Add manual test verifying cache key loaded via CDN

[jacekbogdanski]

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Followup ticket to #5003

It would be good to see whether the cache key behaves correctly when the editor is loaded via CDN. We already have some tests imitating CDN:

/tests/plugins/preview/manual/previewcdn

We should check if we can use the same/similar logic and reuse it. Note that we want to see if CSS files are loaded correctly, so the script in the above manual test may require some update (like removing CSS files before loading CDN).

[CKEditorBot]

Closed in #5035