Add a generateNewTestUser() method to PhabricatorTestCase

Summary: I need this shortly and it seems like something we're likely to need more of in the future now that fixtures work.

Test Plan: Ran unit tests. Used this productively in an upcoming diff.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3775