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.
`IdleTracker` in its construction quietly assumed that the clock it
receives from its parent starts ticking from 0 at the point at which it
is passed down. This is not necessarily the case when headless
executions are involved, which means that the initial state of the
tracker could be computed as idle incorrectly.
Resolve by explicitly reading the clock time at the point of
`LoadComplete()`.