Add test cases and .gitignore

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>