tests: Make schema test fail on XML schema errors

tests: Make schema test fail on XML schema errors

Failure to parse the schema file would not trigger a test suite failure.

In addition to making the test fail it's necessary to split up the
parsing of the schema file into a separate test.

This is necessary as the XML validator uses libvirt errors to report
problems parsing of the actual schema RNG needs to be split out into a
separate function and called via virTestRun which has the
infrastructure to report them.