I think it's worth cleaning up this

```python
        self.vumi_helper = DjangoVumiApiHelper()
        self.add_cleanup(self.vumi_helper.cleanup)
        self.vumi_helper.setup_vumi_api()
```

dance because we do it in a lot of places. The setup_vumi_api can maybe be an argument. Maybe we can have an .add_helper method that takes the helper class and params for __init__ and returns an instance already registerd for clean-up?