Fix failing tests / CI pipeline and clean-up code

Scheduler submodule tests need to be disabled until it's working, currently it breaks CI.

As far as I know, Ad Scheduler module is fine, but missing a composer.json with the dependency for the “scheduler” module. That's why PHPUnit can't find the Scheduler classes. I actually added that in the 11.x meta issue, but it was removed again before merge for some reason.