Don't pop invalid test environment

Summary: If `unset($env)` throws then we pop some other environment instead which is impossible to pop later.

Test Plan:
  $ arc unit src/infrastructure/env/__tests__ src/applications/calendar/storage/__tests__

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4488