Set up GitLab CI

I assigned the task myself because I plan to fix this issue and make a Merge Request.
Please do not work on this issue.

Set up GitLab CI. Please review MR. Thanks!

Do we actually need this? AFAIK we just enable testing in the project node settings.

Tests definied through the "Automated testing" use DrupalCI, which is deprecated. The prefered way to do testing is through the new(ish) gitlab-ci pipeline! https://www.drupal.org/docs/develop/git/using-gitlab-to-contribute-to-dr... →

Tests won't run on the main module page anymore without a .gitlab-ci.yml.