refactor: prefer admin_client fixture over client.login

[afuetterer]

Description

pytest-django offers a admin_client fixture, i.e. a client logged in as an admin user.

Using this fixture reduces the number of "logins" needed in the test suite, offering a tiny performance boost.

It is not "the" rdmo admin (admin:admin), but that was not used in the tests before. So not a problem.

Types of Changes

• [x] Refactoring (no functional changes, no api changes)

Checklist

• [x] I have read the contributor guide.

[jochenklar]

Cool, I was not aware of this.

[afuetterer]

In case we ever need the "admin user" (from the fixture accounts.json) the be logged in, we can override the admin_client fixture.