Fix phantomjs test runner with requirejs

Summary:
The problem is QUnit is loaded before everything else, so it fires the
QUnit.done event before any of the tests have actually ran.

This kind of hacky fix just has the test runner ignore a QUnit.done
event that didn't actually run any tests.

It's kind of ugly but I am not sure if there's a simpler solution. I
also tried moving QUnit into a require in the last callback, but that
caused the test runner to complain that QUnit wasn't present.

Test Plan: * `phantomjs test/runner.js test/index.html`

Reviewers: jack, alpert

Reviewed By: alpert

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D7217