libc++ tests: wait_until.pass test sporadically fails (bug 21998)

Summary:
Hello Howard,

While running the libc++ tests on our ARM boards, we encounter sporadic failures of the two tests:
test/std/thread/futures/futures.shared_future/wait_until.pass.cpp
test/std/thread/futures/futures.unique_future/wait_until.pass.cpp

The worker thread might not finish yet when the main thread checks its result.
I filed the bug 21998 for this case: http://llvm.org/bugs/show_bug.cgi?id=21998

Would you be able to review this please?
Thank you.
Oleg

Reviewers: howard.hinnant, mclow.lists, danalbert, jroelofs, EricWF

Reviewed By: jroelofs, EricWF

Subscribers: EricWF, mclow.lists, aemerson, llvm-commits

Differential Revision: http://reviews.llvm.org/D6750

llvm-svn: 228783