AbstractValidationUnitTest's LSResourceResolver should resolve from the jboss.dist.dir

The AbstractValidationUnitTest class used in the various build/dist modules scans ${jboss.dist.dir}/docs/schema to identify the available xsds and build a schemas map, but then when StandardConfigsXMLValidationUnitTestCase validates our config files the LSResourceResolver it uses does not always look there. If it's asked to resolve a systemId not in its schemas map it tries to resolve using the TCCL. But very few relevant jars are on the test classpath (basically just jboss-metadata-common) so that's pretty pointless. If that doesn't work it should try the same dir it used to build the map.

The logic for all this is repeated in different places, but all of those can validly depend on testsuite/shared as a test dep, so part of this work can be to consolidate the code into testsuite/shared.