Refactor random_page test to use setup
Refactor random_page test to use setup

By separating the stubbing from the request and assertions it's a bit easier
to see what's going on and allows us to provide a short sentence to describe
what the test is observing.

I've also modified some whitespace related to line wrapping.

alphagov/frontend,alphagov/frontend,alphagov/frontend,alphagov/frontend