Add a lint test for audio notifications.

Only traits are linted - the UI still hardcodes
too many audio references for this to be worthwhile.

OpenRA.Mods.Cnc/Traits/Infiltration/InfiltrateForCash.cs

@@ -33,6 +33,7 @@ namespace OpenRA.Mods.Cnc.Traits
 		[Desc("Maximum amount of funds which will be stolen.")]
 		public readonly int Maximum = int.MaxValue;
 
+		[NotificationReference("Speech")]
 		[Desc("Sound the victim will hear when they get robbed.")]
 		public readonly string Notification = null;