How to handle `onConfirm` in grunt-mocha-phantomjs?

Hi there, I was wondering if there is a `grunt-mocha-phantomjs` best practice for using the phantomjs `onConfirm` [callback](http://phantomjs.org/api/webpage/handler/on-confirm.html).  I would like to test some things that trigger `window.confirm`. Maybe writing tests for `window.confirm` isn't really a thing.
