Switch to Wayland instead of X11 for our CI tests

[pcdavid]

Our CI tests, run at https://ci.eclipse.org/sirius, use Xvnc to run a headless X11 session inside a Linux VM.
X11 is being abandonned in favor of Wayland in most/all Linux distributions, so we should probably switch to a headless Wayland env at some point.

The main test job which is run daily at https://ci.eclipse.org/sirius/job/sirius.tests-master uses a free-form configuration with the Xvnc Jenkins plug-in and some additional configuration:

The https://ci.eclipse.org/sirius/job/sirius-pr-check job launched when a PR is created/updated also uses Xvnc but via a Jenkinsfile.

Both will need to be updated.

It looks like it's possible to run mutter (the Wayland compositor from Gnome) in headless mode:

mutter --wayland --headless --virtual-monitor 1920x1080 --no-x11 --wayland-display=wayland-42 --sm-disable

And then launch a (Wayland-compatible) app like this:

WAYLAND_DISPLAY=wayland-42 GDK_BACKEND=wayland terminator

This requires the GNOME 40 (or later) version of mutter: https://www.phoronix.com/news/GNOME-40-Headless-Virtual. Not sure which one (if any) is available in the Docker images provided by CBI.

[pcdavid]

It looks like it's mutter 3.32.2 at the moment. Not sure which version of Gnome this corresponds to.