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