Add automated test coverage that POTX works against all of core

Does https://www.drupal.org/project/potx → project have drush commands that would fail if a situation like the one presented in the (split) issue happens?

It’d be relatively easy to create a new job, if there is a command. We could use (untested):

'🌐 Drupal Translations (potx)':
  <<: [ *with-composer, *default-job-settings-lint ]
  stage: 🪄 Lint
  when: manual
  variables:
    KUBERNETES_CPU_REQUEST: "2"
  script:
    - composer require drupal/potx
    - drush potx ...

Potx does have a drush command, I am not sure it reports errors found in a proper way for testing. It depends on what kind of smoke test we are looking for. If we want to check for misuses of syntax as well, it has false positives which would need to have some kind of baseline to test against then. https://localize.drupal.org/translate/projects/drupal/releases/559203#so... is the 163 warnings currently found. Most of them are likely false positive :)

That said, if we only want a smoke test: whether it fatals or not, that is probably easier :D