We often forget to add new files to POTFILES, and also
have some paths in the skip file that have not existed
for 15+ years. We should ensure that these files include
the paths that should be there, and nothing more.
This commit adds a test that checks whether all the files
in the po lists exist, and vice versa, that all the files
in the source tree that should be included in this list,
are indeed included in this list.