Came up as a failure when locally running tests for
ppy/osu-framework#6001 - but the test is actually a previously-known
flaky that I couldn't reproduce the failure of until the aforementioned
PR.
This appears to be a simple race; the test scene queries the track
length from update thread, but the length is actually set on the audio
thread. So it's not unreasonable that given unlucky timing, the length
will not be set by `TrackBass` before it is queried.
To fix, switch assert to until step. I'm generally not really willing
to give this more time of day until this change is proven insufficient.
