Tests the API in XML mode.

This form of testing has been nearly available for awhile but was not
turned on because it will increase the time of a full tox test run by
eight seconds. However there has recently been new features which do
not work at all with XML, so the time loss seems like a silly reason
to not gate on this.

implements blueprint test-with-xml

Change-Id: I98d892bb5c1a0c16425f80a28a296c70120318c8
Merge "Tests the API in XML mode."