Add a lint for "extern mod extra;" in run-pass tests without "xfail-fast".

[bblum]

aaaaaaargh.

[alexcrichton]

It would be awesome if we could not do the concatenate-all-tests thing on windows. That would allow us to test a lot more functionality to ensure things work on windows.

I presume that we concatenate everything because forking is slow on windows, so there may not be a way around the root problem. I agree though that for now a lint would be nice.

[huonw]

Triage: maybe we could resolve this with extern mod extra; etc. at the top of the crate that the combiner creates.

cc #8844 (check-fast metabug), and #6330 (similar thing, but for std::.. uses).