Remove SelectedRoom from online play tests

[smoogipoo]

• Depends on Remove RoomManager, centralise listing management to lounge #31866

It's one of those "magic" abstractions that I'd prefer to not exist.

In multiplayer tests, if a room is required (generally the case), you can put a:

public override void SetUpSteps()
{
    base.SetUpSteps();
    JoinDefaultRoom();
}

See for example: https://github.com/ppy/osu/blob/205d6ecffbc989d75c1a32e53a29a9342b88c175/osu.Game.Tests/Visual/Multiplayer/TestSceneMultiplayerQueueList.cs

[peppy]

@smoogipoo there's one conflict with a newly added test TestChangeSettingsButtonVisibleForHost could you have a check on that?