Support validating output XML

Add support for validating the output XML using one or more schemas, either XSD, RelaxNG, or Schematron, in any combination.

This would allow us validate kpet output both against the stock Beaker Job RelaxNG schema, and CKI-specific requirements written in Schematron. Support for XSD is added just in case, as another decent schema language.

Output the "error_log" property on validation failure as is for now, even though it looks like something a cat would throw up. It's still possible to figure out what's going on, and the failures shouldn't be frequent. Making better validation output would require much more schema type-specific code.

Goes together with redhat/centos-stream/tests/kernel/kpet-db!884 (merged)