Support validation settings in TDML

See Mike's comment about TDML support for invoking validation, specifically Schematron.

... it seems to me we want to be able to write TDML tests with validation settings that will cause them to invoke schematron as part of the runtime. E.g., define a tdml:testSuite with defaultValidation="schematron" or a specific tdml:parserTestCase with validation="schematron"