Also note the change from `songSelect.ChildrenOfType<>()...` to
`this.ChildrenOfType<>()...` - because the new design is registered at
game-level, the mod select is not a child of the song select screen
anymore.
The old design was apparently somehow tolerant of multiple instances of
the same mod in `SelectedMods`, but the new one is not.
Fix the test, because appending to a list of mods that was getting
modified by previous tests is generally not how tests should be
written.