Improve how built images are tested
This MR includes three commits:
- Deduplicate variable definition in .gitlab-ci.yml
- Add a test pipeline to test ARM build jobs
- Add and sort missing images variables in the test pipeline definition
With this MR, I would like to improve how the images built by the internal pipeline are tested. Currently, only x86 images are tested, and I would like to add support for ARM and the upcoming RISC-V images. This should be then merged before !523 (merged).