`TestSceneBeatmapSkinFallbacks` was locally caching a `GameplayState`
instance to fulfill dependencies of a
`SkinnableTargetComponentsContainer`. However, it was doing so using
`new TestBeatmap()`, which is a raw decoded beatmap that hasn't been
converted to any ruleset yet, which was causing failures in
`BeatmapDifficultyCache.GetTimedDifficultyAttributesAsync()` as that
method is expecting to receive a post-conversion, ready-for-gameplay
beatmap.
Resolve by proxying forward dependency instances from the
already-known-to-be-working `actualComponentsContainer`.
All other similar UI components have required dependencies, so this is
mainly to bring things in line with expectations. I am using this fact
in the skin editor to only show components which can be used in the
current editor context (by `try-catch`ing their
`Activator.CreateInstance`).
