bpo-38546: multiprocessing tests stop the resource tracker (GH-17641) (GH-17647)

Multiprocessing and concurrent.futures tests now stop the resource
tracker process when tests complete.

Add ResourceTracker._stop() method to
multiprocessing.resource_tracker.

Add _cleanup_tests() helper function to multiprocessing.util: share
code between multiprocessing and concurrent.futures tests.

(cherry picked from commit 9707e8e22d80ca97bf7a9812816701cecde6d226)
5 files changed