fix JSON tests which do XML compares to use XMLUnit

The JSON tests have a mode where they generate xmi/xml serializations as well, just for extra coverage. THe compare of these against expected can't be a string "equals" because different XML parsers, transformers, etc. do ordering of name-spaces and formatting of empty elements differently. Use instead the XMLUnit package (available from Maven Central - license OK) (thanks to suggestion from rec), to check for "equality" instead that allows for these differences.