[DMN Editor] Introduce marshalling test in the way DMN - String - DMN

Currently marshaling test assumes as the initial input a *.dmn file that is consequently converted to Graph, next this Graph is converted to XML and finally the XML is transformed to Graph that is asserted for required properties.

This task should introduce test that will assume as an initial input Graph, that will be converted to xml, this xml will be converted again to Graph and finally this Graph will be asserted for required properties.