I found a bug in wpt_batch_lib.py.
To work, it should force the xml output with f=xml parameter.
Can be corrected changing line 70 from:
request = server_url + 'runtest.php?%s' % urllib.urlencode(test_params)to:
request = server_url + 'runtest.php?&f=xml&%s' % urllib.urlencode(test_params)