[#50351081] Add a smokey (read) check for the fact cave

So that we know whether or not the app is responding as we expect it
to, get the fact (VAT rate) rate) and use that to make sure the
application is working.

Currently, we only want to run this smokey check in preview -- hence the
@notproduction flag.
Expect to see the response fields, not a value

It's brittle to expect the description for VAT rate to be consistent.
Better off matching against the fields of the JSON response instead.
Merge pull request #37 from alphagov/initial-fact-cave-smoke-tests

[#50351081] Add a smokey (read) check for the fact cave