`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`.
`TestSceneStatisticsPanel` intends to check the operation of statistics
panels using dummy ruleset classes. However, `StatisticsPanel` relies on
being able to retrieve the playable beatmap, which requires a converter.
One was not provided by the dummy rulesets, therefore the retrieval
would fail with an unobserved exception.
To fix, add a barebones converter implementation that is enough for the
test to pass.