Make BackupDB self contained

It would be nice to test the app as well. After some resarch, I think we could have something like

```shell
DJANGO_SETTINGS_MODULE = "whatever" python setup.py test
```

And we could use [these](https://docs.djangoproject.com/en/dev/topics/testing/advanced/#module-django.test.utils) in a custom test `tearUp` and `tearDown` method.