Fix failing test in PhabricatorPolicyDataTestCase caused by membership locking

Summary: This fixes a unit test failure that started occurring due to the new membership locking feature.

Test Plan: Ran the unit tests.

Reviewers: btrahan, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10546