[PATCH] Workaround ASAN race condition in future tests by manually joining threads.

[Dan Albert]

> I *really* *really* don't like the idea of changing tests to push bugs under the rug. It'd be much better to 'XFAIL: asan' these. I have a thought on how to fix asan that I'll add to the PR.

Agreed.

http://reviews.llvm.org/D6347