bpo-37421: test_concurrent_futures stops ForkServer (GH-14643) (GH-14645)

test_concurrent_futures now explicitly stops the ForkServer instance
if it's running.
(cherry picked from commit e676244235895aeb6ec3b81ca3ccf4a70e487919)

Co-authored-by: Victor Stinner <vstinner@redhat.com>
2 files changed