[3.6] bpo-31249: Fix test_concurrent_futures dangling thread (GH-3521) (#3522)

ProcessPoolShutdownTest.test_del_shutdown() now closes the call queue
and joins its thread, to prevent leaking a dangling thread.
(cherry picked from commit 3bcf157)

Author: miss-islington

Lib/test/test_concurrent_futures.py

@@ -226,11 +226,14 @@ def test_del_shutdown(self):
         list(executor.map(abs, range(-5, 5)))
         queue_management_thread = executor._queue_management_thread
         processes = executor._processes
+        call_queue = executor._call_queue
         del executor
 
         queue_management_thread.join()
         for p in processes.values():
             p.join()
+        call_queue.close()
+        call_queue.join_thread()
 
 
 class WaitTests: