Play-test from a running game returns into the game

Stop brings the player back to the scene they were in instead of the main
menu, and a scene switch inside the played scenario ends the play-test in
place. New GUI test suite ingame_play covers both.

test_suites/ingame_play/src/InGamePlayTestInitializer.cs

@@ -0,0 +1,31 @@
+using System.Linq;
+using Cthangover.Core.Mods;
+using Cthangover.Core.Utils;
+using Godot;
+
+namespace Cthangover.Tools.Tests.InGamePlay
+{
+    /// <summary>
+    /// Entry point of the in-game play-test GUI test, activated only by the
+    /// exact user arg <c>--itest=ingame_play</c>. The driver is added deferred:
+    /// resources-ready fires before the game's shell exists.
+    /// </summary>
+    public class InGamePlayTestInitializer : IModInitializer
+    {
+        private static bool driverQueued;
+
+        /// <summary>Nothing to do per mod: the driver is queued on resources-ready.</summary>
+        public void OnModLoaded(string modId) { }
+
+        /// <summary>Queues the driver once when the activation flag is present.</summary>
+        public void OnModResourcesReady()
+        {
+            if (driverQueued || !OS.GetCmdlineUserArgs().Any(a => a == "--itest=ingame_play"))
+                return;
+
+            driverQueued = true;
+            GameLogger.Log("TEST", "InGamePlayTestInitializer: activating in-game play-test test");
+            (Godot.Engine.GetMainLoop() as SceneTree)?.Root?.CallDeferred("add_child", new InGamePlayTestDriverNode());
+        }
+    }
+}