Fix roundtrip-test crasher

The first issue was that I left out the interwiki prefix because I thought it
wasn't necessary anymore (I was wrong), and the second issue was that
xmlFormat now requires an env object (though it only uses the page.name from
there), so I faked it for the case of the roundtrip test runner, where we will
not ever need a full env object in the XML callback.

Change-Id: I3d441692da07788def3676931212400f6a4a1d36