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.

Lib/test/_test_multiprocessing.py

@@ -4740,6 +4740,8 @@ class TestSyncManagerTypes(unittest.TestCase):
             self.proc.terminate()
             self.proc.join()
         self.manager.shutdown()
+        self.manager = None
+        self.proc = None
 
     @classmethod
     def setUpClass(cls):