As seen: https://github.com/ppy/osu/pull/17252/checks?check_run_id=5545717506
The `PlaylistsResultsScreen` takes a lease on the `Beatmap` bindable
when entered. During `SetUp`, the `Beatmap` bindable is reassigned but
fails in cases where an existing test instance of the screen hasn't been
exited yet. This fix moves the assignment into the `SetUpSteps` function
after `base.SetUpSteps` is called which will exit the existing screens
first.
The main goal here is to remove the inheritance, since realm doesn't
like that. Unfortunate that we can't use object initialisers in a few of
these places, but no real way around that.