Use os.cpu_count in test_runner instead of multiprocessing
Summary:
This eliminates unnecessarily importing multiprocessing. multiprocessing.cpu_count() just calls os.cpu_count() anyway:
https://github.com/python/cpython/blob/main/Lib/multiprocessing/context.py#L43
The or 1 keeps the linter happy since os.cpu_count() can return None in some rare cases.
Test Plan:
ninja check-functional
Reviewers: O1 Bitcoin ABC, #bitcoin_abc, PiRK
Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D17327