Cleanup integration tests prototype usage, naming and unused variables (#3114)

* Fix naming of prototypes variable in tests

* Remove unused variables in tests

* Remove unused variables in tests

* Replace content entities with test entities

* Fix airlock and lung test

Content.IntegrationTests/Tests/DummyIconTest.cs

@@ -27,7 +27,7 @@ namespace Content.IntegrationTests.Tests
                 {
                     if (!proto.Components.ContainsKey("Sprite")) continue;
 
-                    SpriteComponent.GetPrototypeTextures(proto, resourceCache).ToList();
+                    var _ = SpriteComponent.GetPrototypeTextures(proto, resourceCache).ToList();
                 }
             });
         }