I guess the changes that involved `MultiplayerTestScene` having a test
user lookup cache caused this test case to false-pass silently.
Added an explicit assert which ensures the added user indeed has a null
`User` value.
`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).