Address conflict with Features over config.installer class

Added a PHPUnit test, however it requires a composer.json to add dev dependencies (features.)

The two composer jobs are failing due to the lack of a Drupal 11 compatible version of features.
Personally, I think having the ability to test interoperability between this module and features is valuable, however if features doesn't create a compatible release, we'll be stuck with broken jobs.

We could consider just copying the parts of features we need to test into our test module.

trackleft2 → changed the visibility of the branch 3.0.x to hidden.