CI takes a very long time: ~1h , ideally should complete in 15min

When I run the full test suite on my laptop, it takes from 10 to 20 minutes to run the full job so long as I have the sources precached (which they should be with our gitlab runners).

Running the full test suite on gitlab takes approximately one hour; sometimes it can take close to 2 hours (this happens rarely, happened yesterday).

My laptop is powerful developer laptop with 4 cores (8 threads) running at 3.8GHz... with plenty of RAM and SSD.

The tests only take one core at a time anyway; and while my laptop is powerful for a laptop, it's nothing compared to a "powerful CI server".

Tests should run faster in CI than they do on a developer laptop; the whole thing (all jobs) should ideally complete in 15min, more than 30min is very, very bad.

Emmet has taken an action point in the last meeting to coordinate the effort and plan the right meetings to ensure we setup a proper machine to run CI with.

Current fixes / time savings:

• @valentindavid's !715 (merged): ~20min
• @tristanvb's !715 (merged): ~3min
• Fixing the gitlab cache (#603 (comment 96922243)): ~13min
• Move code_quality job to test stage (!737 (merged)): ~3min

Current pipeline times:

• ~24min (total): https://gitlab.com/BuildStream/buildstream/pipelines/28736295
  • tests jobs take ~18min (from those ~2min are for caching)