Bug 1075475 - [CostControl] Fix unit tests with a newer mocha r=salva r=...

...djf
>  https://github.com/mozilla-b2g/gaia/blob/master/dev_apps/test-agent/common/test/sinon_helper.js.

Sorry, this was the sinon helper :) The "done" change is https://github.com/mozilla-b2g/gaia/blob/master/dev_apps/test-agent/common/test/mocha_generators.js#L36-L49