CI: split pmbootstrap tests from building packages?

I really like the pipeline system in gitlab, where it is able to show tests or failures for smaller parts than the overall pipeline (unlike Travis, where is only one status for the whole thing).

In my opinion, it would be helpful if we split the tests that build changed aports into its own pipeline step.

CC: @craftyguy (in case you should have time for this, but the non-working CI issues are more important to me in case you would like to spend your time on any of it)