Use formencode's xml_compare method to compare test results. This solves test failures on several systems.

@lrowe would you mind doing a quick review of my pull request? I would like to move our Jenkins slaves to the latest Ubuntu 14.04 and currently almost all diazo tests fail.
I was too hasty okaying this earlier. These tests check that script tags are parsed correctly - in HTML that is valid markup and we should ensure it works.