Disable XML tests on Py26

Unit testing on Py26 takes too long to run.  The code paths are the
same, so we'll temporarily skip tests is the class contains XML
formating since duplicate full stack JSON tests are run.

Change-Id: Ie8eddfe0b6af39f32380ce532e0c26acc0a6596d
Related-Bug: 1295281
Merge "Disable XML tests on Py26"