Move helpers/setup into partial

This is the first step towards breaking down the tests. I’ve had to do some
funny things to ensure all references stay up-to-date – I’m not sure if there’s
a cleaner way. IIRC you can also stick things on Mocha’s `this`, which is shared
between `it`/`before`, etc.

I wonder if this could clean things up on that front:
http://bitcubby.com/writing-selenium-tests-with-node-the-enjoyable-way/?utm_source=nodeweekly&utm_medium=email