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

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.

Lib/multiprocessing/util.py

@@ -439,3 +439,28 @@ def close_fds(*fds):
     """Close each file descriptor given as an argument"""
     for fd in fds:
         os.close(fd)
+
+
+def _cleanup_tests():
+    """Cleanup multiprocessing resources when multiprocessing tests
+    completed."""
+
+    from test import support
+
+    # cleanup multiprocessing
+    process._cleanup()
+
+    # Stop the ForkServer process if it's running
+    from multiprocessing import forkserver
+    forkserver._forkserver._stop()
+
+    # Stop the ResourceTracker process if it's running
+    from multiprocessing import resource_tracker
+    resource_tracker._resource_tracker._stop()
+
+    # bpo-37421: Explicitly call _run_finalizers() to remove immediately
+    # temporary directories created by multiprocessing.util.get_temp_dir().
+    _run_finalizers()
+    support.gc_collect()
+
+    support.reap_children()