Make tests pass again

Problem/Motivation

We missed a deprecation when getting ready for Drupal 11 https://www.drupal.org/node/3284397 →
So after 📌 Remove deprecated code from bootstrap and lib/Controller, lib/Config Fixed got committed tests on Drupal 11 started failing.

Steps to reproduce

run tests

Proposed resolution

fix \Drupal\Tests\config_split\Kernel\SplitTestTrait

Remaining tasks

we might as well also clean up other things that prevent MRs from being green.

User interface changes

none

API changes

none

Data model changes

none