Run the test suite against Drupal 10, the next minor and Drupal 12

Problem/Motivation

The pipeline only runs against the current core version. The module declares support for Drupal 10.3, Drupal 11 and Drupal 12, and nothing checks the two ends of that range.

That gap is not theoretical for this module. It carries code that behaves differently depending on the PHPUnit version underneath: the test declares its group with both a PHP attribute and an annotation, because PHPUnit 9 reads only the annotation and PHPUnit 12 reads only the attribute. Exactly one of those two mechanisms is exercised on any given run, so a single version pipeline can only ever prove half of it.

The Drupal 12 support declared recently is in the same position: it was established by reading the code and confirming that no deprecated core API is used, which is good evidence but not a passing test run.

Proposed resolution

Turn on the three opt in variables the drupal.org GitLab templates provide: OPT_IN_TEST_PREVIOUS_MAJOR for Drupal 10, OPT_IN_TEST_NEXT_MINOR for the upcoming minor, and OPT_IN_TEST_NEXT_MAJOR for Drupal 12. The templates already define the jobs; the project only has to ask for them.

This is deliberately a separate change from adding the pipeline in the first place, and it comes after the Drupal 12 declaration rather than before, so that each pipeline result answers one question.