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.Common/Traits/Player/HarvesterAttackNotifier.cs

@@ -26,6 +26,7 @@ namespace OpenRA.Mods.Common.Traits
 		[Desc("Length of time (in ticks) to display a location ping in the minimap.")]
 		public readonly int RadarPingDuration = 10 * 25;
 
+		[NotificationReference("Speech")]
 		[Desc("The audio notification type to play.")]
 		public string Notification = "HarvesterAttack";