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.
A few ideas:
* these specific tests (along with the `pretty-print = false` one) could be implemented separately and use xml_compare for the rest.
* If the `pretty-print = false` test does not currently fail with the new libxml2 then maybe use the existing comparison for that and make these tests also `pretty-print = false`.
* Instead of using xml_compare we could normalize the xml ourselves before comparison when not using `pretty-print = false`. (Strip out all whitespace, replace '>' with '>\n'.)