bpo-33789, test_asyncio: Hide PendingDeprecationWarning (GH-7461)

Hide PendingDeprecationWarning in test__register_task_3().
diff --git a/Lib/test/test_asyncio/test_tasks.py b/Lib/test/test_asyncio/test_tasks.py
index 1c1a0ac..1079d49 100644
--- a/Lib/test/test_asyncio/test_tasks.py
+++ b/Lib/test/test_asyncio/test_tasks.py
@@ -2613,7 +2613,8 @@
         self.assertEqual(asyncio.all_tasks(loop), set())
         self._register_task(task)
         self.assertEqual(asyncio.all_tasks(loop), set())
-        self.assertEqual(asyncio.Task.all_tasks(loop), {task})
+        with self.assertWarns(PendingDeprecationWarning):
+            self.assertEqual(asyncio.Task.all_tasks(loop), {task})
         self._unregister_task(task)
 
     def test__enter_task(self):