Expand docs to include setting up Grocer.pusher for testing

I wanted to add tests for grocer but found they kept hanging until I set up `Grocer.pusher` differently for `Rails.env.test`. I expect other people have had the same issue so thought it might be worth expanding the docs.
I might change the advice to mention that in [test environments, Grocer will default to a local gateway (IP `127.0.0.1`)](https://github.com/grocer/grocer/blob/892bb8429ab83b31538dd3834dd46b1e7b770550/lib/grocer/push_connection.rb#L30-L31), and doesn't require a certificate. Though I do wonder, what was "hanging" when you provided a gateway and cert? And what environments were those for?