Test-only job

Migrated issue

Reported by: fjgarlin

Related to !153 (merged) !121 (merged)

Problem/Motivation

Try to create a test-only job, similar to what core does.
- For D7:
https://git.drupalcode.org/project/drupal/-/blob/7.x/.gitlab-ci/pipeline.yml?ref_type=heads#L150-160
https://git.drupalcode.org/project/drupal/-/blob/7.x/.gitlab-ci/pipeline.yml?ref_type=heads#L101-130

- For D10:
https://git.drupalcode.org/project/drupal/-/blob/11.x/.gitlab-ci.yml?ref_type=heads#L146-160
https://git.drupalcode.org/project/drupal/-/blob/11.x/.gitlab-ci/pipeline-test-only.yml?ref_type=heads
https://git.drupalcode.org/project/drupal/-/blob/11.x/.gitlab-ci/scripts/test-only.sh?ref_type=heads

Steps to reproduce

We currently have no test-only job

Proposed resolution

Try to mirror how core does it.

Related issue: Issue #3425373

Related issue: Issue #3425971

Related issue: Issue #3389921