gh-109649: Fix test_os.test_process_cpu_count_affinity() (#111689)

When CPUs are isolated on Linux, os.process_cpu_count() is smaller
than os.cpu_count(). Fix the test for this case.

Example with "isolcpus=5,11 rcu_nocbs=5,11" options passed to a Linux
command line to isolated two logical CPUs:

$ ./python -c 'import os; print(os.process_cpu_count(), "/", os.cpu_count())'
10 / 12