Improve Salsa CI pipeline's own CI to help avoiding regressions in the pipeline code

I would describe better this issue in the future, but I just want to record our own CI needs to be improved. I'd like to verify the different features, at least the most important, to avoid breaking stuff.

Some initial questions, that could be translated to other issues:

• are the build options/argument correctly handled?
• aptly useful in downstream pipelines?
• test-build-{any,all} run correctly?
• reprotest running all the variations correctly?
• changes in salsa-ci-team/autopkgtest-lxc

This would require work in .pipeline-test.yml to create an extensive set of pipeline test. And we maybe don't need to run them all the time to optimise use of resources.