Automated testing: Configure GitLab CI

Quick follow-up on this ticket:

As suggested in the Issue summary, added at #2 initial configuration file .gitlab-ci.yml based on template and documentation from Documentation on Drupal.org: GitLab CI → , to provide automated testing with GitLab CI pipelines.

After seeing the first pipeline being run automatically at:
https://git.drupalcode.org/project/image_link_formatter/-/pipelines/17847
The job for phpcs came back with a warning and coding standards errors, see:
https://git.drupalcode.org/project/image_link_formatter/-/jobs/74233
which were fixed in the following commit at #3.

and the following pipeline on the second commit came back all green (passed), see:
https://git.drupalcode.org/project/image_link_formatter/-/pipelines/17850

The new interface on GitLab provides a lot more information on the builds and on the tests as well, see:
https://git.drupalcode.org/project/image_link_formatter/-/pipelines/1785...

While providing a lot more control over the testing infrastructure, it also requires quite a lot more configuration work to port existing testing jobs variables from DrupalCI.
I haven't looked yet at the details of the DrupalCI Variables, but it looks like we should be able to run tests with the same configurations we had on DrupalCI.
I have given it a quick test by changing the core version $CORE_SUPPORTED to 9.5.x, which seemed to work quite well and downloaded current 9.5.10, see in composer logs:
https://git.drupalcode.org/project/image_link_formatter/-/jobs/74258#L282

I haven't found yet how more jobs could be added to be triggered automatically on commits or patches in issues.
I suppose this will most likely be taken care of in merge requests, issue forks branches on the GitLab side.
Otherwise, another option would be to create Scheduled pipelines, which could run certain tests on a time basis, like every week, month and such.

For the time being, the default configuration has been enabled for the 2.1.x, which should trigger tests automatically on commits in the branch and most likely merge requests. More tests have to be triggered manually, for example for various versions of Drupal Core, PHP, MySQL, etc... and we should be able to improve the configuration as we get a better understanding of the features provided in GitLab CI.

Lastly, as it currently stands, it doesn't really seem necessary or interesting to backport support for GitLab CI Testing to the 2.0.x branch of the module, which was marked not recommended and will become unsupported at some point.

Since all the work to be carried in this ticket should have been completed at this point, marking it as Fixed for now.

Feel free to let us know if you have any questions, suggestions or comments on any of the recent commits or the project in general, we would be very happy to hear your feedback.
Thanks in advance!

Automatically closed - issue fixed for 2 weeks with no activity.