Not the most obvious keys, but does match stable expectations so let's
go with it for now.
Rationale for not using +/- is that local audio offset was bound to
those, and people will expect it to be when we get to implementing
eventually.
Also, adjust the scroll speed is a pretty rare thing to do since it's
been constant across beatmaps for years now.
The order of operation of imports means that the already-imported
instances are considered most correct (they have usually been populated
using an online source, although that is skipped in tests), so on two
consecutive test imports in a test scene, the second will lose its
online IDs completely.
This aims to ensure that all test beatmaps will have online IDs, which
is the general expected behaviour.