Allow tests to be skipped

Summary:
Allow tests to be skipped by calling assertSkipped(). It's not really
an assertion of anything tangible; more like "assert that we can't
really assert anything right now".

Test Plan: Added a new test to the PhutilUnitTestEngineTestCase.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2312