Add feature to allow test dependencies to be installed at Next Major

In gitlab_templates_downstream#3571809 we have the functionality to allow test dependency modules to be installed in Next Major phpunit jobs. This allows the project to be tested at Next Major. Without this, the phpunit jobs fail to start if any of their dependencies are not compatible. This defeats the object of testing at Next Major.

As per this suggestion, instead of just having this feature internally in the Downstream test branches, it could be added to Gitlab Templates, allow any Contrib project to make use of it.