Run loop tests with more loop classes

[almarklein]

• The majority of tests in test_loop.py can now also be applied to QtLoop.
• Same for WxLoop, but the tests don't work, because the mechanics that we need to use to work around wx's idiosyncrasies prevent that.
• QtLoop is tested when PySide6 is installed.
• Can force a particular loop using e.g. pytest tests/test_loop.py -k PyQt6Loop
• Would be awesome to run these tests on CI, but Qt aborts and wx needs a full build so 🤷
• CI now does do a test that simply attempts to import the backend module (for PySide6, PyQt6 and PyQt5).
• CI now also runs the test_gui_glfw.py (actually creating a glfw window and render something with wgpu).