RFC: Move libcore/libstd tests into src/test/run-pass

[erickt]

As nice as it is having the tests in the source code, it's pretty painful trying to fix a bug in libcore/libstd, and have to go through at least one full stage compile to see if I fixed it. It would be much faster if we migrated the tests into src/test/run-pass.

[thestinger]

@erickt: I usually just compile core.rc or std.rc with a previous compiler build and --cfg stage2 --test.

It's definitely annoying to have mtime bumped for editing code that runs in a separate stage of the compile though.

[thestinger]

You can compile std.rs, extra.rs, syntax.rs and rustc.rs individually now with rustc --test since the hacks with the naming of std and extra are gone. I think this has become a non-issue, but please re-open if you disagree.