Fix randomly failing subscribe unsubscribe test (#1008)

Add a delay before asserting topic is unsubscribed in unsubscribe test

Author: bjsowa

rosbridge_library/test/internal/subscribers/test_multi_subscriber.py

@@ -93,6 +93,7 @@ def test_subscribe_unsubscribe(self):
         self.assertEqual(len(multi.new_subscriptions), 0)
 
         multi.unregister()
+        time.sleep(0.1)
         self.assertFalse(is_topic_subscribed(self.node, topic))
 
     def test_subscribe_receive_json(self):