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.
Stubbing the request in this manner will return an `WebMock::RequestStub` and not an `GdsApi::Response` object. This might lead to problems when using the helper. Maybe it would be beneficial to follow the implementation of the other two methods, modifying it slightly?