Make beatmap carousel select nearest to selection when selection was filtered

[LittleEndu]

• Supersedes Don't select random beatmap when previous was filtered #14499

This PR changes carousel behaviour when currently selected beatmap gets filtered. Right now the selection is random, this PR would change that to be nearest to previous selection. This PR is basically previous PR but tests fixed.

Changes

• Make BeatmapCarousel use PerformSelection when previous selection was filtered (instead of SelectNextRandom)
• Make CarouselGroupEagerSelect perform selection in both direction, selecting nearest to previous selection (instead of only selecting forwards)
• Remove test case that was testing previous random behaviour
• Write new test case for new behaviour

[peppy]

@LittleEndu I've rewritten the main method here to avoid using LINQ, mostly because the logic you had in the LINQ was basically duplicating the non-LINQ logic for the edge case scenarios.

I think this looks good otherwise, but please check my implementation to make sure I haven't (well, the tests haven't) overlooked anything.

[LittleEndu]

Looks good to me.

[peppy]

I've fixed the remaining test failure but i'm seeing occasional failures on TestAddRemove. I'm not sure if it's related to this change.

[LittleEndu]

Real failure seems to be that during testing/debugging, carousel root sometimes skips eager selection, and then TestAddRemove would succeed. But if it actually performs eager selection, then its last selected index would be bigger than Items.Count so the new LINQ free solution would just return null. Changing the line to be int backwardsIndex = lastSelectedIndex == Items.Count ? Items.Count - 1 : lastSelectedIndex; would fix and makes sense if deleting last beatmap of the carousel (which the test does)

[LittleEndu]

Suggested change

	int backwardsIndex = lastSelectedIndex;
	int backwardsIndex = lastSelectedIndex == Items.Count ? Items.Count - 1 : lastSelectedIndex;