fix test tasks

Author: kumaraditya303

Lib/test/test_asyncio/test_tasks.py

@@ -2272,10 +2272,8 @@ async def kill_me(loop):
 
         self.assertEqual(self.all_tasks(loop=self.loop), {task})
 
-        asyncio._set_event_loop(None)
-
         # execute the task so it waits for future
-        self.loop._run_once()
+        self.loop.run_until_complete(asyncio.sleep(0))
         self.assertEqual(len(self.loop._ready), 0)
 
         coro = None