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/Buildings/ProductionAirdrop.cs

@@ -23,7 +23,9 @@ namespace OpenRA.Mods.Cnc.Traits
 	[Desc("Deliver the unit in production via skylift.")]
 	public class ProductionAirdropInfo : ProductionInfo
 	{
+		[NotificationReference("Speech")]
 		public readonly string ReadyAudio = "Reinforce";
+
 		[Desc("Cargo aircraft used for delivery. Must have the `Aircraft` trait.")]
 		[ActorReference(typeof(AircraftInfo))] public readonly string ActorType = "c17";