Fix trade tariff search tests

The old tests used a broken URL that redirected to a 200. Smokey's default behaviour is to follow this URL and get the final status. So, fix those URLs.

Secondly, even if the search returned no results (as was the case with preview etc. recently) these tests would pass because of the 200 status on the search controller. So update (and refactor while doing so) the tests to look for success text on the page.