tests: cleanup GraphicsEngine to prevent shutdown freeze on windows
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.

chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d