Fix potential nullref in TestSceneAutoplay check steps method

[frenzibyte]

Since LoadTrack is now called on constructor in tests, the audio manager is necessary to have in order to construct a virtual track for the beatmap. TestSceneAutoplay however dodgily creates a working beatmap in a non-loaded context just to use it for time reference on the seek step descriptions. Have replaced by only creating the IBeatmap, which is enough for time reference.

I've also updated a few methods that combine the AddStep on the same line as the method declaration, which made it somewhat hard to tell that the code is being executed inside the step because of the indentation difference.