Scenario play-test returns to the screen the editor was opened from

The play-test loads the game shell, which frees the screen the editor was
opened from (the novel_editor hub). Both the scenario's end and Stop used to
switch to the engine's main menu; the editor now remembers the route it left
and switches back to it, falling back to the main menu only when it was not
opened from a route. The tools_hub run of the windows suite plays its own
scenario and checks both ways back.

test_suites/windows/scenarios/playtest_return.scenario

@@ -0,0 +1,5 @@
+scene: home_basement
+priority: 0
+---
+delay 60 hidden=true
+end