StandardConfigsXMLValidationUnitTestCase uses wrong AbstractValidationUnitTest parent class from base EAP code base

In XP codebase the test class `org.wildfly.dist.subsystem.xml.StandardConfigsXMLValidationUnitTestCase` uses as parent class `org.wildfly.test.distribution.validation.AbstractValidationUnitTest` which is taken from base EAP code base. This means that any change in testing requires changes in base EAP. However, there is also a class `org.wildfly.dist.subsystem.xml.AbstractValidationUnitTest` in the same package as `org.wildfly.dist.subsystem.xml.StandardConfigsXMLValidationUnitTestCase` which is not used at all which makes me think that it was intended for this purpose. We should refactor `org.wildfly.dist.subsystem.xml.StandardConfigsXMLValidationUnitTestCase` to use `org.wildfly.dist.subsystem.xml.AbstractValidationUnitTest` so the changes required for EAP XP are part of the xp branches.