cargo-miri: support running unit tests for libraries as well as test binaries

[apoelstra]

No description provided.

[apoelstra]

This currently won't work because of #479 but it can be tested by hacking in a separate attribute that Miri detects rather than #[test].

[RalfJung]

TBH, I don't actually understand this code, but if CI is happy it can't be worse than the old one.^^ What do you think @oli-obk?

[apoelstra]

To clarify -- for tests we pass --test <testname> -- <rustc args> to cargo. For libraries we pass -- --test <rustc args> to cargo. That is, we're moving --test from a cargo argument (find an appropriate test binary, compile it, etc etc) to a rustc argument (compile the test runner for whatever you're already being told to compile).

[RalfJung]

If you you jet down some comments into the code, that'd be nice. :D