Test helper for getting sub-sector organisations

Add this test helper, so that we can test that the collections app is
displaying the list of organisations associated with content in a
sub-sector.
The problem with this approach is that I'd need to be able to get hold of the search_client object in the caller.  eg, in whitehall you use the existing helpers by calling Whitehall.unified_search_client.stubs(...).  However, in collections (where I want to use this), I can't see any way to get hold of the search client that's going to be used.

Instead, I modelled the stub on the way the content api ones are done (eg, https://github.com/alphagov/gds-api-adapters/blob/master/lib/gds_api/test_helpers/content_store.rb#L20 ), which is just to mock the web request. This makes the tests in `collections` stub things in a consistent way (eg, https://github.com/alphagov/collections/blob/organisations-on-subsector-pages/test/controllers/specialist_sectors_controller_test.rb#L53 ).

I'm open to suggestions, but this does at least work! :)