Test we can destroy INVALID_ENTITY

This is part of the spec and wasn't unit-tested for.

Differential Revision: https://code.wildfiregames.com/D1735
This was SVN commit r22036.

source/simulation2/tests/test_Simulation2.h

@@ -1,4 +1,4 @@
-/* Copyright (C) 2017 Wildfire Games.
+/* Copyright (C) 2019 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -107,6 +107,10 @@ public:
 
 		sim.FlushDestroyedEntities(); // nothing in the queue
 
+		// Allow destroying INVALID_ENTITY which should do nothing.
+		sim.DestroyEntity(INVALID_ENTITY);
+		sim.FlushDestroyedEntities();
+
 		sim.DestroyEntity(ent2);
 		sim.FlushDestroyedEntities(); // already deleted