Remove twig_theme_test.module

Moved the function to a static method, can't use a trait as it's used in a test theme, not a test module.

The Needs Review Queue Bot → tested this issue. It fails the Drupal core commit checks. Therefore, this issue status is now "Needs work".

This does not mean that the patch necessarily needs to be re-rolled or the MR rebased. Read the Issue Summary, the issue tags and the latest discussion here to determine what needs to be done.

Consult the Drupal Contributor Guide → to find step-by-step guides for working with issues.

Fixed phpcs.

Seems like a good conversion to me.