The intention here is to make things more testable going forward.
Specifically, to remove the "back-door" entrance into `BeatmapCarousel`
where `BeatmapSets` can be set by tests and bypas/block realm retrieval.
The old design was apparently somehow tolerant of multiple instances of
the same mod in `SelectedMods`, but the new one is not.
Fix the test, because appending to a list of mods that was getting
modified by previous tests is generally not how tests should be
written.