Add test code to ensure python#91887 is fixed

Author: alexhartl

Lib/test/test_asyncio/test_tasks.py

@@ -2279,6 +2279,14 @@ async def kill_me(loop):
         source_traceback = task._source_traceback
         task = None
 
+        # no more reference to kill_me() task in user code. the task
+        # should be kept alive as long as it is pending and we hold a
+        # reference to the event loop (#91887)
+        support.gc_collect()
+
+        self.assertEqual(len(asyncio.all_tasks(loop=self.loop)), 1)
+        mock_handler.assert_not_called()
+
         # remove strong reference held by the event loop
         self.loop._pending_tasks.clear()