Return top-priority category as parent for top-level page.

This should resolve gma/nesta#147.
When there's only one test that needs some setup data I don't favour a `describe` block and before block any more. I may have used this pattern in Nesta's tests in the past (I've not checked), but if I have I'll be moving away from it in future.

In other words, I favour just creating what's required in an `it` block and using local variables, unless the data created is used in multiple tests.

I'm only mentioning it because these tests will obviously need to change anyway, given my comments above – not because I'm a total zealot! ;-)