Allow PhabricatorEnv to be temporarily made mutable for unit tests

Summary: Introduces a scope-guarded way to override the env config, for unit tests which are sensitive to config values.

Test Plan: Ran unit tests.

Reviewers: btrahan, vrana, jungejason

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D2237