Uses the resolved working beatmap instead of resolving it every time.
Also uses the EditorBeatmap itself as playable beatmap, as it is of type `IBeatmap` already, and `.PlayableBeatmap` forwards everything anyway.
This is different from the working beatmap's `.Beatmap` property in that it is mutated by the ruleset/editor.
So hit objects, for example, are actually of type `Slider` and such instead of the legacy `ConvertSlider`.
This should be preferred over `workingBeatmap.Beatmap`.
This lets us access things like the background, track, etc. which are necessary for quality and filesize checks.
Also improves the structure of the `CheckBackgroundTest` class in the process.
