Move TestCase descriptions to attributes

Depends on https://github.com/ppy/osu-framework/pull/1186.

osu.Game/Tests/Visual/TestCasePlayer.cs

@@ -23,8 +23,6 @@ namespace osu.Game.Tests.Visual
 
         protected Player Player;
 
-        public override string Description => @"Showing everything to play the game.";
-
         /// <summary>
         /// Create a TestCase which runs through the Player screen.
         /// </summary>