[OpenMP] Use half of available logical processors for collapse tests (#88319)

The new collapse test cases define `MAX_THREADS` to be 256 and use all
available threads/logical processors on the system. This triples the
testing time on an AIX machine that has 128 logical processors. This
patch changes to use half of available logical processors to avoid over
subscribing because there are other libomp tests running at the same
time, including 2 other such collapse tests.