Rewrite tests.mk in Rust

[brson]

All the Rust tests use the Rust test runner. The Makefile for the test suite is one big macro for configuring the Rust test runner. It's self contained, and needs to be rewritten for the next build system.

Integrate with @graydon's build system. It may make sense to put useful bits in std::test.

[nikomatsakis]

Not critical for 0.6; removing milestone

[bstrie]

@catamorphism , does this sound like something that's now the purview of rustpkg?

[emberian]

Visiting for triage; not all the tests use the official test runner, a bunch of them use compiletest etc... I'm certainly in favor of slaughtering tests.mk but it'd probably need to be done with the rest of the codebase when it's ported to rustpkg (though it could probably be ported first, or something?)

/cc @catamorphism

[brson]

rustpkg will not be sufficient to replace tests.mk. between compiletest and the doc tests, it does a lot

[brson]

and compiletest and the doc tests do use the rust test runner

[metajack]

triage bump. nothing to add.

[huonw]

part of #8058

[treeman]

Triage bump.

[brson]

I don't have any particular desire to rewrite the build system in rust these days. Bootstrapping all the things doesn't really hold the same allure to me any more. When we rewrite the Rust buildsystem I'll probably be more inclined to do it in Python.