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.
Use parens for calls with long parameter names

Without using parens, for long parameter names, it's easy to miss the
space and get confused.
Merge pull request #193 from alphagov/rummager-test-helper-sector-organisations

Test helper for getting sub-sector organisations

https://www.pivotaltracker.com/story/show/75807888