Problem/Motivation
Since the parent issue has been committed to D9+, maintaining a project compatible with 8.9 and 9+ has become problematic. D9 now reports a lot of deprecations, but those cannot be fixed as D8 version of the base test classes define $modules with public visibility.
Steps to reproduce
See for example https://www.drupal.org/pift-ci-job/1853861 (migration-related tests cannot be fixed)
Proposed resolution
Backport changes to 8.9.x
Remaining tasks
Patch review
User interface changes
None
API changes
None
Data model changes
None
Release notes snippet
TBD