Unsupported tests for foreign items

[adpaco]

We have two tests in:

rust-tests/cbmc-reg/ForeignItems/fixme_main.rs
rust-tests/cbmc-reg/ForeignItems/fixme_varadic.rs

[adpaco]

These could be related to #72 since one of the tests seems to work with variadic code.

[adpaco]

fixme_varadic.rs works if called as kani fixme_varadic.rs --c-lib lib.c (I think the kani-flags are wrong)
fixme_main.rs does not work because of a single assertion. I think this is either a bug in the implementation or the test itself, and needs to be reviewed.

[zhassan-aws]

C-lib support is still experimental. Removing "MLP" label.