Add very basic functional testing to CI

This will test the basic functionality of both standard and keyscript modes in CI. It works by creating a fake X11 display using xvfb and emulating keypresses on a qemu image based encrypted disk.

Due to this, errors in the keyboard itself will not be found by this script. Rather only errors in unlocking and keyscript output will be discovered.

EDIT: After a few retries, it looks like testing standard mode won't work in docker as it needs to be able to set up new block devices. This test will not run if it detects GitLab CI