Refs #18586 -- Split m2m_signals test

I don't think we need `self.expected_messages` to be a class attribute -- how about this instead:
```
expected_messages = [{
    ....
}, {
    ....
}]
self.assertEqual(self.m2m_changed_messages, expected_messages)
```
self.assertEqual(self.m2m_changed_messages, expected_messages)