To test upscroll easier, and with all parts in conjunction.
`ManiaSkinnableTestScene`s already had the capability to switch
directions, but they did not show all parts together, which meant
regressions were missed.
Makes it hard to test anything because `EditorBeatmap`'s `Update` method
updates whether a beatmap has timing or not (enabling the placement
controls).
Also adds a basic timing point to allow for better testing.