fix: restore asyncio designator for async tests

Author: vchudnov-g

tests/asyncio/test_operation_async.py

@@ -83,7 +83,6 @@ async def test_constructor():
     assert future.metadata is None
     assert await future.running()
 
-
 @pytest.mark.asyncio
 async def test_metadata():
     expected_metadata = struct_pb2.Struct()