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

...djf
you'll see I use `this.sinon` everywhere now. `this.sinon` is a sandbox (managed by the code in https://github.com/mozilla-b2g/gaia/blob/master/dev_apps/test-agent/common/test/sinon_helper.js) and as a result it's automatically restored at teardown. This makes the test code easier to read :)