Added some additional assertion methods.

Summary:
There are quite a few tests in Arcanist, libphutil and Phabricator that do something similar to `$this->assertEqual(false, ...)` or `$this->assertEqual(true, ...)`.

This is unnecessarily verbose and it would be cleaner if we had `assertFalse` and `assertTrue` methods.

Test Plan: I contemplated adding a unit test for the `getCallerInfo` method but wasn't sure if it was required / where it should live.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8460