Add coverage for test-only changes job

We could somehow alter a file using the before_script and maybe we also need to override the rules section for the "test-only job" on one of the files so it always runs as well.

Yes we can alter files in before_script but even if we changed the rules to trigger the job, I am not sure that this "diff" would pick up the changes when the pipeline is not running from a Merge Request. It might do, so worth a try. But also altering the rules kind-of invalidates the overall testing of the job.

I will try the above, but it might end up that we create a special MR here, marked as draft, which remains open and we run it in a new downstream pipeline (with other jobs skipped so we don't duplicate and hence waste resource each time)

We can only do it on one branch, or just do it for the parent issue (the one of failing curl) and then revert the change.
Or we can add a new rule that will trigger it, like setting a variable, which we could set in the gitlab_templates project when calling the downstream pipelines.

Something like this:

  rules:
    - if: '$FORCE_TEST_ONLY_JOB == "1"'
       when: always
    - *opt-in-current-rule
    - *skip-test-only-changes-rule
    - changes:
        - '**/tests/**/*Test.php'
      when: manual