`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`.
https://github.com/django/django/blob/233165083593baf5f62194f50df26fb830e5f570/django/contrib/auth/backends.py#L11