Automated testing: Configure GitLab CI

Quick follow-up on this issue:

The initial GitlabCI template was added at #2 and the tests can now correctly run on GitLabCI, see:
https://git.drupalcode.org/issue/queue_unique-3426116/-/pipelines/113079

The corresponding merge request !MR5 came back green ✅ with two warnings:

• CSpell, with a minor typo and some unrecognized names in the Test files. The typo could be fixed easily, but I'm not sure exactly how the warnings could be fixed for the strings in the tests, see:
  

  ./tests/src/Kernel/UniqueQueueTest.php:110:16    - Unknown word (testq)      -- $names = ['testq1', 'myotherq2'];
  	 Suggestions: [test, testa, tests, testy, tets]
  ./tests/src/Kernel/UniqueQueueTest.php:110:26    - Unknown word (myotherq)   -- names = ['testq1', 'myotherq2'];

  Should the strings be modified, or is there a way to ignore/skip these strings/errors?

• PHPCS errors which are most likely addressed in 📌 Fix the errors/warnings reported by Php_CodeSniffer Needs review

 
Otherwise, the changes as they currently are in the merge request should fix this issue and allow integrating module's tests with GitLabCI.

We would greatly appreciate if a maintainer could please take a look at the changes in this ticket, in particular the corresponding merge request !MR5 and give us some comments and feedback.

Feel free to let us know at any time if you have any questions or concerns on any of the changes in the merge request or this ticket in general, we would surely be glad to help.
Thanks in advance!

✅ Fixed all errors and warnings above and the tests all seem to be passing (green! 🟢) now \o/
https://git.drupalcode.org/issue/queue_unique-3426116/-/pipelines/115105

Updated the documentation of merge request MR !5 with the additional changes to fix cspell and phpcs validation errors.

Thanks in advance for your feedback !