Run cross-platform commands for headless GUI testing

This is similar to https://github.com/GabrielBB/xvfb-action (at least in purpose), but with a few differences:

1. runs on node16, silencing GitHub deprecation warnings
2. doesn't use xvfb-run, instead runs xvfb once for all commands
3. by default installs and starts a minimal window manager (see pytest-qt troubleshooting for info on why)
4. allows installing additional linux dependencies and customizing setup/teardown commands