Add FIXME comment in an integration test

Author: seratch

integration_tests/webhook/test_async_webhook.py

@@ -93,6 +93,7 @@ async def test_with_unfurls_on(self):
         time.sleep(2)
         history = await client.conversations_history(channel=self.channel_id, limit=1)
         self.assertIsNotNone(history)
+        # FIXME: when repeatedly running this test, the following assertion can fail
         self.assertTrue("attachments" in history["messages"][0])
 
     @async_test