`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`.