This test case was incorrect, it was relying on the fact
that the function we were calling would continue to sleep
for the requested time even if it was interrupted.  That is
not true of std::this_thread::sleep_for, at least not on OS X.

Fix the test case so that if it wakes up early, it goes back
to sleep till the time is actually greater than the end point.

<rdar://problem/18523742>