futures-channel tests do not work out of the tree

[igor-raits]

If you download crate file from crates.io and will try to run cargo test, it would fail with:

+ /usr/bin/cargo test --release -j8 --no-fail-fast
       Fresh either v1.5.0
       Fresh futures-core v0.2.1
   Compiling futures-channel v0.2.1 (file:///home/brain/rpmbuild/BUILD/futures-channel-0.2.1)
     Running `/usr/bin/rustc --crate-name futures_channel src/lib.rs --emit=dep-info,link -C opt-level=3 --test --cfg 'feature="default"' --cfg 'feature="futures-core"' --cfg 'feature="std"' -C metadata=a1733f59a9578215 -C extra-filename=-a1733f59a9578215 --out-dir /home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps -L dependency=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps --extern futures_core=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps/libfutures_core-dcf311156a5a594c.rlib -Copt-level=3 -Cdebuginfo=2 -Clink-arg=-Wl,-z,relro,-z,now`
     Running `/usr/bin/rustc --crate-name channel tests/channel.rs --emit=dep-info,link -C opt-level=3 --test --cfg 'feature="default"' --cfg 'feature="futures-core"' --cfg 'feature="std"' -C metadata=f0b7cdb79b11e0d1 -C extra-filename=-f0b7cdb79b11e0d1 --out-dir /home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps -L dependency=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps --extern futures_core=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps/libfutures_core-dcf311156a5a594c.rlib --extern futures_channel=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps/libfutures_channel-42fe9e30c037da53.rlib -Copt-level=3 -Cdebuginfo=2 -Clink-arg=-Wl,-z,relro,-z,now`
     Running `/usr/bin/rustc --crate-name mpsc tests/mpsc.rs --emit=dep-info,link -C opt-level=3 --test --cfg 'feature="default"' --cfg 'feature="futures-core"' --cfg 'feature="std"' -C metadata=edfbd2dcb430fcc4 -C extra-filename=-edfbd2dcb430fcc4 --out-dir /home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps -L dependency=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps --extern futures_core=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps/libfutures_core-dcf311156a5a594c.rlib --extern futures_channel=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps/libfutures_channel-42fe9e30c037da53.rlib -Copt-level=3 -Cdebuginfo=2 -Clink-arg=-Wl,-z,relro,-z,now`
     Running `/usr/bin/rustc --crate-name mpsc_close tests/mpsc-close.rs --emit=dep-info,link -C opt-level=3 --test --cfg 'feature="default"' --cfg 'feature="futures-core"' --cfg 'feature="std"' -C metadata=20d0fcfb59f0d7a1 -C extra-filename=-20d0fcfb59f0d7a1 --out-dir /home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps -L dependency=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps --extern futures_core=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps/libfutures_core-dcf311156a5a594c.rlib --extern futures_channel=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps/libfutures_channel-42fe9e30c037da53.rlib -Copt-level=3 -Cdebuginfo=2 -Clink-arg=-Wl,-z,relro,-z,now`
     Running `/usr/bin/rustc --crate-name oneshot tests/oneshot.rs --emit=dep-info,link -C opt-level=3 --test --cfg 'feature="default"' --cfg 'feature="futures-core"' --cfg 'feature="std"' -C metadata=dfb63aa120c9a7a7 -C extra-filename=-dfb63aa120c9a7a7 --out-dir /home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps -L dependency=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps --extern futures_core=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps/libfutures_core-dcf311156a5a594c.rlib --extern futures_channel=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps/libfutures_channel-42fe9e30c037da53.rlib -Copt-level=3 -Cdebuginfo=2 -Clink-arg=-Wl,-z,relro,-z,now`
error[E0463]: can't find crate for `futures`
 --> tests/mpsc-close.rs:1:1
  |
1 | extern crate futures;
  | ^^^^^^^^^^^^^^^^^^^^^ can't find crate
error[E0463]: can't find crate for `futures`
 --> tests/channel.rs:1:1
  |
1 | extern crate futures;
  | ^^^^^^^^^^^^^^^^^^^^^ can't find crate


error: aborting due to previous error

For more information about this error, try `rustc --explain E0463`.
error: aborting due to previous error

For more information about this error, try `rustc --explain E0463`.
error[E0463]: can't find crate for `futures`
 --> tests/oneshot.rs:1:1
  |
1 | extern crate futures;
  | ^^^^^^^^^^^^^^^^^^^^^ can't find crate

error: aborting due to previous error

For more information about this error, try `rustc --explain E0463`.
error[E0463]: can't find crate for `futures`
 --> tests/mpsc.rs:2:1
  |
2 | extern crate futures;
  | ^^^^^^^^^^^^^^^^^^^^^ can't find crate

error: aborting due to previous error

For more information about this error, try `rustc --explain E0463`.
error: Could not compile `futures-channel`.

Caused by:
  process didn't exit successfully: `/usr/bin/rustc --crate-name mpsc_close tests/mpsc-close.rs --emit=dep-info,link -C opt-level=3 --test --cfg feature="default" --cfg feature="futures-core" --cfg feature="std" -C metadata=20d0fcfb59f0d7a1 -C extra-filename=-20d0fcfb59f0d7a1 --out-dir /home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps -L dependency=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps --extern futures_core=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps/libfutures_core-dcf311156a5a594c.rlib --extern futures_channel=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps/libfutures_channel-42fe9e30c037da53.rlib -Copt-level=3 -Cdebuginfo=2 -Clink-arg=-Wl,-z,relro,-z,now` (exit code: 101)
warning: build failed, waiting for other jobs to finish...
error: Could not compile `futures-channel`.

Caused by:
  process didn't exit successfully: `/usr/bin/rustc --crate-name channel tests/channel.rs --emit=dep-info,link -C opt-level=3 --test --cfg feature="default" --cfg feature="futures-core" --cfg feature="std" -C metadata=f0b7cdb79b11e0d1 -C extra-filename=-f0b7cdb79b11e0d1 --out-dir /home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps -L dependency=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps --extern futures_core=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps/libfutures_core-dcf311156a5a594c.rlib --extern futures_channel=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps/libfutures_channel-42fe9e30c037da53.rlib -Copt-level=3 -Cdebuginfo=2 -Clink-arg=-Wl,-z,relro,-z,now` (exit code: 101)
warning: build failed, waiting for other jobs to finish...
error: Could not compile `futures-channel`.

Caused by:
  process didn't exit successfully: `/usr/bin/rustc --crate-name oneshot tests/oneshot.rs --emit=dep-info,link -C opt-level=3 --test --cfg feature="default" --cfg feature="futures-core" --cfg feature="std" -C metadata=dfb63aa120c9a7a7 -C extra-filename=-dfb63aa120c9a7a7 --out-dir /home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps -L dependency=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps --extern futures_core=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps/libfutures_core-dcf311156a5a594c.rlib --extern futures_channel=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps/libfutures_channel-42fe9e30c037da53.rlib -Copt-level=3 -Cdebuginfo=2 -Clink-arg=-Wl,-z,relro,-z,now` (exit code: 101)
warning: build failed, waiting for other jobs to finish...
error: Could not compile `futures-channel`.

Caused by:
  process didn't exit successfully: `/usr/bin/rustc --crate-name mpsc tests/mpsc.rs --emit=dep-info,link -C opt-level=3 --test --cfg feature="default" --cfg feature="futures-core" --cfg feature="std" -C metadata=edfbd2dcb430fcc4 -C extra-filename=-edfbd2dcb430fcc4 --out-dir /home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps -L dependency=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps --extern futures_core=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps/libfutures_core-dcf311156a5a594c.rlib --extern futures_channel=/home/brain/rpmbuild/BUILD/futures-channel-0.2.1/target/release/deps/libfutures_channel-42fe9e30c037da53.rlib -Copt-level=3 -Cdebuginfo=2 -Clink-arg=-Wl,-z,relro,-z,now` (exit code: 101)
warning: build failed, waiting for other jobs to finish...
error: build failed