A lot of tests are using test resources that populate the length field,
but do not populate hitobjects. The general expectation is that
components should be using the cached length in cases where hitobjects
are not relevant, but `GetVirtualTrack` was doing its own local
calculation.
This could cause tests to fail due to `MusicController` changing track
in the background.
The import process was running on the async load thread, but then
accessed from the access thread later on. This seemed to somehow pass
fine in headless runs, but would fail on visual test execution
(specifically on `TestBeatmapConfirmed()`).