tests: Check for orphaned layout files

This adds a test which scans the layouts directory for files which have not been added to the resources file. The majority os user-submitted layouts don't include layout registration, and I ran out of ideas for how to make it more visible in the documentation. Instead, it's going to be solved on the other side: the CI pipelines shall fail.

To make it easy on experimenters with untidy trees, it's disabled by default.