Split .gitlab-ci/testcases/test_aports.py in multiple files

pmaports.git used to be in the same repository as pmbootstrap.git. Therefore, we have this file .gitlab-ci/testcases/test_aports.py which originally only held test cases relevant to pmaports. We keep adding new tests to this file. But now that the whole repository is about pmaports, let's split it up into multiple files (grouped together as it seems useful, I wouldn't mind one file per test).

This can be done after !1759 (closed) is merged (didn't want to delay #865 (closed) by adding this to the MR).