Add tests for resizing images in CKEditor 5

Problem/Motivation

In #3348603-28: CKEditor 5 resizes images with % width instead of px width (the CKEditor 4 default): breaks image captions *and* is a regression → , we tried to write test coverage for resizing images. However, this test was failing at least on my machine locally so we decided not to commit the test coverage.

Proposed resolution

Make sure tests pass on several local environments and DrupalCI before committing the tests.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet