PHPUnit tests are never run in GitLab CI" (Task, 3.0.x).

Problem/Motivation

PHPUnit job always passes without running any tests, which misleads contributors
Since #3547679, .gitlab-ci.yml replaces the phpunit job script with an echo: "Skipping PHPUnit tests - see issue for test failures"

This is very confusing for developers:
The project has Functional, FunctionalJavascript and Kernel tests, and merge requests show a green phpunit job
Reviewers and maintainers get the same false signal when they decide whether a merge request is safe to merge.

Proposed resolution

Restore the default phpunit job so that CI results mean something again