gitlab-ci: Distribute jobs across more stages

gitlab-ci: Distribute jobs across more stages

The stages and mapping of jobs to them are somewhat arbitrary; the goal
is to avoid having to scroll through large numbers of jobs.

v2: (Pierre-Eric Pelloux-Prayer)
* Use even more stages for test jobs
* Give somewhat meaningful names to stages

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3995>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3995>