Setup CI/CD with both test reports and coverage on PHPUnit 10.x

To see it and almost not believe it:

• Drupal core-dev, phpstan, and all of their dependencies will conspire to prevent you from using PHPUnit 10.x, since they all require PHPUnit < 9.x.
• Can't install PHPUnit 10.x at the Drupal root (breaks drupal-recommended), or at the contrib module (breaks Gitlab's Composer merge ops).
• No PHPUnit 10.x === no coverage, but I was having none of that.

Resolution:

• Installed PHPUnit 10.x directly in the Gitlab Docker container, overriding the default Gitlab CI template for phpunit.
• Locally, some type hinting is still needed for coverage to be user-friendly, so added a new tools/phpunit directory with the desired PHPUnit and respective composer.json, that way the new PHPUnit does not grief Drupal core or Gitlab CI.

About those coverage stats now... well, that's a battle for another day, but the groundwork has been done at the Gitlab level.

Pix or didn't happen: