Make file storage more testable

Summary: Fixes T2474. Adds a storage dummy storage engine for unit tests, and adds a couple of simple tests for basic file storage.

Test Plan: Ran `arc unit` to execute unit tests.

Reviewers: kwadwon

Reviewed By: kwadwon

CC: aran

Maniphest Tasks: T2474

Differential Revision: https://secure.phabricator.com/D4777