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/PlaceBeacon.cs

@@ -18,7 +18,10 @@ namespace OpenRA.Mods.Common.Traits
 	public class PlaceBeaconInfo : ITraitInfo
 	{
 		public readonly int Duration = 30 * 25;
+
 		public readonly string NotificationType = "Sounds";
+
+		[NotificationReference(typeFromField: "NotificationType")]
 		public readonly string Notification = "Beacon";
 
 		public readonly bool IsPlayerPalette = true;