Add way for testsuite to pull in schematron from use-cases repo

[macintoshpie]

Would be nice if users of the testsuite could just use the schematron files without having to download them. This would encourage folks to manage their schematron in our use-case repo as well

[macintoshpie]

Alternatively, (probably preferably) we just add an example for downloading files that come from the use-case repo and how to use them with the testsuite tool