Minor change to ArcanistInfrastructureTestCase

Summary: Update ArcanistInfrastructureTestCase after D9860 and D9861.

Test Plan: arc unit should do the trick.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9863