Fix non-determinism in tests.

The stdlib's xml apparently cares about ordering but also doesn't
preserve it. There are a number of different recommendations of other
ways to do this, but they all depend on packages we don't have
available (lxml or xml.dom.ext, in particular).

Test: pytest
Bug: None
Change-Id: I07108d8977c302404e7c90ca75a4bf7a1144750f