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.
Content API does indeed do it this way which is fine for things like Cucumber where you don't interrogate objects directly, but I've found it to be a stumbling block if you use these in specs.

If you're OK stubbing and using mocks in your units etc then it should be fine to only mock the http request, but it needs to be a conscious decision to make these acceptance-only helpers.