Refactor searcher unit tests.

Instead of testing for a particular search body, test each individual
aspect of it separately, so we can tell more easily what's changed, and
we don't break existing tests by adding a new parameter to the search.
Request 50 results per search.

By default, elasticsearch returns 10 results, which isn't enough.
Returning 50 is consistent with the list pagination.
Merge pull request #55 from alphagov/moar-search-results

Raise the limit on search results