Similar to `Section` in the timing screen, but does not make use of checkboxes, nor specific to control points.
So there's a lot of things that differ, hence new class instead of factoring that out.
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.