Quit test drivers through ApplicationExit

Drivers finish through ApplicationExit.Quit so engine mods complete their
asynchronous cleanup before the engine shuts down; failure codes are kept.

test_suites/site_shots/src/SiteShotsDriver.cs

@@ -266,7 +266,7 @@ namespace Cthangover.Mods.CthangoverGame.Tests
         {
             step = -1;
             GameLogger.Log("TEST", $"[TEST_RESULT] name=site_shots passed={passed} failed={failures.Count}");
-            GetTree().Quit(failures.Count == 0 ? 0 : 1);
+            Cthangover.Core.Bootstrap.ApplicationExit.Quit(GetTree(), failures.Count == 0 ? 0 : 1);
         }
     }
 }