Add tests for Schematron against instance files to the transpile test suite

[martindholmes]

We currently validate our generated RNG and test it against both valid and invalid instance files; and we validate our Schematron output. But we don't yet test our Schematron output against valid and invalid instance files. This should be part of the test process.

[martindholmes]

Summary of the approach we're going to take:

1. @sydb will write an XSLT that takes the SVRL output from a validation operation and extracts from it only the error message text, and then sequences all the error messages alphabetically, providing a text file we can use for diffing against expected results.
2. We will add Schematron valid and invalid structures to the existing valid and invalid test files so that we don't have to proliferate tests files, and particularly, we don't have to differentiate between files intended for rng test and files intended for Schematron tests.

[martindholmes]

I think this is done, but we need to take a closer look. Number 1 above is XSLT/svrl2msgs.xslt; and there are tests in the no_tei_content test folder using it. Do we need to do more?