This test assumes that the number of available threads is not 3,
otherwise `#pragma omp parallel` and `#pragma omp parallel
num_thread(3)` are naturally going to do the same thing.
Instead use `omp_get_max_threads() - 1` as the number of threads in the
initial `omp parallel num_thread(N)` and then check that the number of
threads does not match the value in the later `omp parallel`.
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