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

[Marshall Clow]

>>! In D6347#5, @jroelofs wrote:
>   *  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.

I, too, would **much** prefer a fix to ASAN, and/or a way to XFAIL them.
[ But I did note that this is an "intermittent failure" under ASAN ]

http://reviews.llvm.org/D6347