Move to InstallerConfigDirectoryTestBase

Problem/Motivation

See https://www.drupal.org/node/3460001 →

This currently makes the next minor (and next major) jobs fail: https://git.drupalcode.org/project/config_overlay/-/jobs/2126969

Proposed resolution

So we need to move to uncompressed tarballs, which is great. Not sure if we can somehow implement BC for getConfigTarball() by creating a temporary tarball and then just switch out the class name via sed in a .gitlab-ci.yml before_script.

Alternatively, we could just not run those tests on older Drupal, but that will also need a before_script to actually remove the files, because we won't reach setUp() if we extend a class that does not exist in older Drupal.

Maybe just doing a version check at the top of the test file will in fact work, though, not sure.

Remaining tasks

User interface changes

API changes

Data model changes