tests: cleanup GraphicsEngine to prevent shutdown freeze on windows

This occurs with the multithreaded render pipeline.  It's not an issue when using ShowBase because ShowBase also explicitly calls this during shutdown.

tests/display/conftest.py

@@ -20,6 +20,9 @@ def graphics_engine():
     engine = GraphicsEngine.get_global_ptr()
     yield engine
 
+    # This causes GraphicsEngine to also terminate the render threads.
+    engine.remove_all_windows()
+
 
 @pytest.fixture
 def window(graphics_pipe, graphics_engine):