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.
`IHasComboColours` was already mutable (via a strange
`AddComboColours()` method) and exposing a straight list is easier to
work with. `IHasCustomColours` is also similarly externally mutable (in
a way which is not easily removable).