JUnit XML output: Include timezone information in XML

JUnit writes local-time without a timezone by default.  This means that
JUnit XML parsers default to local-time.  The portal doesn't know what
timezone the user of stbt-rig wants so these timestamps can be interpreted
incorrectly.  This change includes timezone information in the XML output.

There is a small chance of incompatibilty caused by this change.  If you
experience this please get in contact at support@stb-tester.com.

stbt_rig.py

@@ -839,7 +839,8 @@ def list_results(self):
 
     def list_results_xml(self):
         r = self.portal._get(
-            '/api/v2/results.xml', params={'filter': 'job:%s' % self.job_uid})
+            '/api/v2/results.xml', params={'filter': 'job:%s' % self.job_uid,
+                                           'include_tz': 'true'})
         r.raise_for_status()
         return r.text