Add test coverage for missing filter format when an editor config entity is saved

Problem/Motivation

#3463524: editor_post_update_sanitize_image_upload_settings fails on missing text format added code to work around the case when the filter format config has been deleted but is still referenced from an editor config entity as this caused upgrade issues. Unfortunately the test that issue added was only possible on 10.6.x because of PHPUnit changes.

Proposed resolution

Add missing test coverage to main.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet