WIP: Make the CI system run regression tests

Albert's special regression test suite finds a lot of bugs in (my) merge requests. For that, he has to run it by hand on the merge requests he cares about (at least that's how I understand it). I consider this a waste of his time and attention.

At the same time, pretty much everything needed to have automatic regressions tests is available:

• The CI infrastructure of GitLab
• The regression testing code in poppler/regtest
• A (small) set of test files in https://gitlab.freedesktop.org/poppler/test

This patch adds a CI pipeline that runs the poppler/regtest code on the test files in the above repository. The patch is in a very early state: I am neither a python nor a CI expert, the regtest code is slightly underdocumented, and it is difficult to test CI pipelines locally. I will need a fair bit of mentoring here. For a start I would be happy if the pipeline would simply render all the files in the test repo. That would only catch programs crashes, but that's at least a step in the right direction.

You may object that this new pipeline isn't worth much because there are so few files in the test repository. However, I keep seeing simple test files attached to issues that could be added easily to the test repo. That repo may not reach the completeness of Albert's test suite in a while, but (I hope that) the CI system running regression tests will motivate people to submit more test files.