[AIRFLOW-5740] Fix Transient failure in Slack test (#6407)

* [AIRFLOW-5740] Fix Transient failure in Slack test

The transient failure is caused by Dict Ordering

* We were comparing string

We were comparing strings

(cherry picked from commit 401003e)

tests/contrib/hooks/test_slack_webhook_hook.py

@@ -104,7 +104,7 @@ def test_build_slack_message(self):
         message = hook._build_slack_message()
 
         # Then
-        self.assertEqual(self.expected_message, message)
+        self.assertEqual(self.expected_message_dict, json.loads(message))
 
     @mock.patch('requests.Session')
     @mock.patch('requests.Request')