add gitlab-ci for testing merge requests

As far as I can tell, the tests are run only on master on jenkins instance. I think it would be nice to run them on every MR too. The setup is derived from https://invent.kde.org/sysadmin/ci-tooling/-/tree/master/invent templates.

Downsides is that it takes a long time to launch the CI job with the current invent setup where the workers are busy.