Improve test-coverage for handling custom error pages

based upon the new kernel test, added https://git.drupalcode.org/project/lupus_ce_renderer/-/merge_requests/47

I think that MR is fine, I just added a minor comment on a method that could be generalized a bit and used in other contexts/tests as well.

I do not see your comment, did you miss submitting it?

Anyway, thank you for the review! Merging then!

@fago: https://git.drupalcode.org/project/lupus_ce_renderer/-/merge_requests/47...