Add tests to methods that produce links to ensure they still work

[ST-DDT]

All methods that produce links pointing to actual resources should have a test ensuring the link resolves with a HTTP-200 (redirects to an actual page are allowed).

This mainly applies to the image module, other modules have to be checked for those links.

The tests ensure we notice broken methods sooner (prior to a release).

Originally posted by @ST-DDT in #3109 (comment)