Explicitly call __destruct() in PhabricatorEnvTestCase

Summary:
Required in order to run tests successfully in the HipHop interpreter.
Similar to D2362.

Test Plan: Run the tests in an HipHop runtime.

Reviewers: epriestley

Reviewed By: epriestley

CC: jungejason, aran, Koolvin, vrana

Differential Revision: https://secure.phabricator.com/D2365