chore: fix test that was waiting before initiating operation

Author: vchudnov-g

tests/unit/test_bidi.py

@@ -828,11 +828,13 @@ def test_rpc_callback_fires_when_consumer_start_fails(self):
         bidi_rpc._start_rpc.side_effect = expected_exception
 
         consumer = bidi.BackgroundConsumer(bidi_rpc, on_response=None)
+
+        consumer.start()
+
         # Wait for the consumer's thread to exit.
         while consumer.is_active:
             pass
 
-        consumer.start()
         assert callback.call_args.args[0] == grpc.StatusCode.INVALID_ARGUMENT
 
     def test_consumer_expected_error(self, caplog):