Quit test drivers through ApplicationExit and free test resources

Adds tests for freeing the temporary ext_resource sub-scene instance; the
animated foreground tests dispose their frame textures.

test_suites/gdscript/src/GdscriptTest.cs

@@ -123,7 +123,7 @@ namespace Mods.DialogBase.Tests
         {
             done = true;
             GameLogger.Log("TEST", $"[TEST_RESULT] name={ResultName} passed={passed} failed={failed}");
-            GetTree().Quit(failed == 0 ? 0 : 1);
+            Cthangover.Core.Bootstrap.ApplicationExit.Quit(GetTree(), failed == 0 ? 0 : 1);
         }
     }
 }