Logic is shared with the timeline blueprints which also have the same
problem of displaying text on top of a combo colour.
Slightly modified the formula. Seems to yield better results on a
subjective check.
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.
Makes use of the new `BeatmapSet.GetPathForFile` method and removes dependency on `WorkingBeatmap` specifically, allowing us to switch to `IWorkingBeatmap` later.
This is used in several places, and so should probably have a function rather than remaining as duplicated code.
Also applies this together with the previous commit to `BeatmapManagerWorkingBeatmap`.
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.