Those of you keeping an eye on KSFileUtilities, will probably have already noticed that the project recently gained a branch for unit testing. For those of you not, well, you've just been told.
So far we're only testing the -[NSString ks_pathRelativeToDirectory:] method. I certainly plan to add more in the future, but for now this is pretty good since it's the most error-prone method in the library, and the foundation of -ks_stringRelativeToURL:.
Of course if you have a little spare time, adding some tests of your own is much appreciated. See the README for instructions on how the repo is laid out.