Improve testing of the module

Problem/Motivation

We currently have some fairly solid Behat tests baked into the module, but there are 2 issues we should resolve:

1. They don't cover all use cases (in particular, the non-states-api cases introduced by ✨ Make use of States API optional Active )
2. They don't run in DrupalCI

Remaining tasks

• Add test cases to cover ✨ Make use of States API optional Active
• Introduce .gitlab-ci.yml similar to what we did in to ensure these tests run in CI →
• Add other linting and code quality tools included in Workflow ECA (by virtue of standard DrupalCI templates?)