Enable check of marshalling in DataSourcesTestCase

https://github.com/jbossas/jboss-as/pull/982 reintroduces checking the difference between the original and the marshalled subsystem xml. This method in DataSourcesSubsystemTestCase should be removed

    @Override
    protected void compareXml(String configId, String original, String marshalled) throws Exception {
        //FIXME remove this and marshall properly
    }

The differences aren't very big between the marshalled and original xml so if it can be fixed it would be good to enable this check.