Do not retain references to processes and managers in TestSyncManagerTypes (GH-11801)

Keeping references to processes and managers between tests makes them count as dangling processes.
(cherry picked from commit 613f729e5d)

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>

Lib/test/_test_multiprocessing.py

@@ -4610,6 +4610,8 @@ def tearDown(self):
             self.proc.terminate()
             self.proc.join()
         self.manager.shutdown()
+        self.manager = None
+        self.proc = None
 
     @classmethod
     def setUpClass(cls):