Check if users activated the 'Allow commits from members who can merge
to the target branch' option in their MRs.
Add the "only" parameter to each job in .gitlab.yml, so the pipeline
can properly run in a "merge request specific context" and give us the
environment variable that contains the MR ID.
Copy all test cases from the pmbootstrap repository, that are actually
testing the aports. They were all adjusted to work nicely in this
repository, together with a brand new set of gitlab-ci configs.
This also includes the changes from this merge request, that had a
better detection of changed packages:
<https://gitlab.com/postmarketOS/pmbootstrap/merge_requests/1621>
