minor: Avoid do_test_message.

This function is a bad idea, as it leads to a possible situation
where you aren't actually testing anything:

    def do_test_message(self, msg: Message, expected_message: Optional[str]) -> None:
        if expected_message is not None:
            self.assertEqual(msg.content, expected_message)

Unfortunately, it's called deep in the stack in some places, but
we can safely replace it with assertEqual here.

zerver/lib/test_classes.py

@@ -1040,7 +1040,7 @@ class WebhookTestCase(ZulipTestCase):
         sender = kwargs.pop('sender', self.test_user)
         msg = self.send_json_payload(sender, self.url, payload,
                                      stream_name=None, **kwargs)
-        self.do_test_message(msg, expected_message)
+        self.assertEqual(msg.content, expected_message)
 
         return msg