`extern mod` doesn't work in windows test runner

[huonw]

Presumably since the tests are just concatenated together as submodules.

(I guess there is already an issue for this, but I couldn't find it.)

[pnkfelix]

visiting for triage, email from 2013-08-12.

I'm wondering if there should be a metabug for "issues that stem from the combine-the-modules testing strategy we used on windows." (decided to take the plunge and make that metabug)

[pnkfelix]

Part of #8844.

[huonw]

Closing as a dupe of #7416.