Make ckeditor.js resource reference context-relative (with patch, +fixed test)

I'm reusing the CKEditorPropertyPlugin in a place with a different URL, and the resource reference to ckeditor.js is currently not context-relative, so it only works on ${context-root}/. All other cms plugins I have tested so far do not expose this problem.

I fixed that in the attached patch (applies against 4.0.3).

I also updated the associated unit test accordingly and fixed a pre-existing bug in it, that made it pass unconditionally.