gh-108388: Split test_multiprocessing_spawn (GH-108396)
Split test_multiprocessing_fork, test_multiprocessing_forkserver and
test_multiprocessing_spawn into test packages. Each package is made
of 4 sub-tests: processes, threads, manager and misc. It allows
running more tests in parallel and so reduce the total test duration.
(cherry picked from commit aa9a359ca2)
Co-authored-by: Victor Stinner <vstinner@python.org>
