Update Lib/test/test_asyncio/test_taskgroups.py

Author: graingert

Lib/test/test_asyncio/test_taskgroups.py

@@ -1081,6 +1081,8 @@ async def throw_error():
                 tg.create_task(throw_error())
         except* MyError:
             pass
+        else:
+            self.fail("should have raised one MyError in group")
 
         await asyncio.sleep(0)