[OpenMP][Tests] Test omp_get_wtime for invariants

As discussed in D108488, testing for invariants of omp_get_wtime would be more
reliable than testing for duration of sleep, as return from sleep might be
delayed due to system load.

Alternatively/in addition, we could compare the time measured by omp_get_wtime
 to time measured with C++11 chrono (for portability?).

Differential Revision: https://reviews.llvm.org/D112458