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 allows us to use the same method of checking for other files that should exist, for example the audio file.
By using the same method, they all share test cases too.
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.
