Closes https://github.com/ppy/osu/issues/31098.
Low effort fix because it was already half broken. The test was testing
in isolation but in actual editor usage it wasn't working as expected.
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.