PerseusApi Tests: onChangeFocus

Summary: Tests for the renderer's onChangeFocus api method

Test Plan:
 * make test && make jest
 * Also put `assert(false)`s into the deferAsserts blocks to verify
   that they were being executed

Reviewers: charlie, joel

Reviewed By: joel

Differential Revision: http://phabricator.khanacademy.org/D10277