Fix a failing XML unit test

Summary: Ref T7215. This test case is consistently failing locally (with a LIBXML_VERSION of 20902).

Test Plan: arc unit

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7215

Differential Revision: https://secure.phabricator.com/D12706