Run a short respdiff test for all merge requests

Now that the respdiff tests on main can detect memory leaks, it is worth running them for every merge request.

The way this is currently implemented makes GitLab CI run three respdiff tests for tags, scheduled pipelines, etc., but I do not see a lot of harm in that, given that the new one only runs for a few minutes.

Despite the fact that the #3054 (closed) goodies are only available in main, I think it would not hurt to backport this MR all the way to v9.16; after all, running a short respdiff test for all code changes may be useful for catching protocol-breaking changes even if we do not detect memory leaks at the same time.

Because of the YAML cleanups also introduced in this MR, it has two prerequisites in other repositories:

• isc-private/bind-qa!44
• isc-projects/images!185