This was causing multiple issues with masking and sizing and really
didn't need to exist in the first place. Also not sure why the pool was
nested inside the scroll container, but it isn't any more. Probably for
the best.
The "un-filter" step causes a `SelectNextRandom` invocation. If this happens to select a difficulty in set 3 other than the previously buffered difficulty #2, the subsequent test would fail.
I've split this test out to remove the random element, but added a new assert to ensure buffered (previously visited?) difficulty is re-selected on return to the same set.
Due to non-deterministic test failures in TestSceneBeatmapCarousel,
migrate the checkSelected helper step from AddAssert to AddUntilStep.
This adds more leniency for performance-related issues while still
checking the desired behaviour.