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

...djf
> I don't doubt this is the correct way to do it but where is this function parameter referred in the docs of mocha?

Actually it's not standard mocha but implemented in https://github.com/mozilla-b2g/gaia/blob/master/dev_apps/test-agent/common/test/sinon_helper.js. We should really take the time to upstream this to mocha.

> If you're using arrow functions after, why not here as well?

I can :) I'm not always logical and methodical :p