GNOME OS: Allow testing GNOME Merge Requests

This is a tracking issue.

The end goal is about using the GNOME OS + openQA test infrastructure in modules, not just in gnome-build-meta. Using gnome-shell and mutter as a test case.

Steps:

• Initial CI jobs for Mutter + GNOME Shell which build + publish sysexts using GNOME OS OCI dev image
  • Demo in tchx84/gnome-shell!9 (closed)
  • Final:
    • ci: Build and publish system extensions (gnome-shell!3390 - merged) • Marge Bot
• Initial CI job to run openQA and deploy sysext onto GNOME OS
  • Demo in tchx84/gnome-shell!9 (closed)
• Initial set of openQA tests in GNOME Shell source tree
  • Demo in tchx84/gnome-shell!9 (closed)
• Update Freedesktop and GNOME CI templates as needed, final PR to update Mutter + GNOME Shell CI config
  • citemplates!45 (merged)
• Document and promote feature
  • Promotion: https://www.codethink.co.uk/articles/2024/A-new-way-to-develop-on-Linux/