Allow configuration of testsuite such that StandardConfigsXMLValidationUnitTestCase can deal with CI testing of a core snapshot with a new kernel xsd

StandardConfigsXMLValidationUnitTestCase fails CI tests of core whenever the core snapshot under test includes a new version of the kernel xsd. See, for example, https://github.com/wildfly/wildfly-core/pull/1366. The problem is the test picks up the new xsd version as the latest, and rejects full's standard config files for using an old xsd.

This makes doing xsd updates in core a hassle.

Goal is to have a way to configure CI runs for core snapshots in full to avoid this problem. System property most likely.

I'm quite certain my fix will be hacky!