`BaseAdminIntegrationTestCase` should use `USERNAME_FIELD`

Username login field can be customised in Django. That means the `User` model wouldn't have necessarily a `username` field. Probably best to rely on `USERNAME_FIELD`.
I would use [`get_username()`](https://docs.djangoproject.com/en/dev/topics/auth/customizing/#django.contrib.auth.models.AbstractBaseUser.get_username).