Aggregations and compositions are not just collaborations; they are part of the class internal design and should not be exposed; not even to tests.
Recently a friend discovered this blog. She asked me why do I write so much about Expressive Design. I will try to give some overview of the topic and link to the many articles in this blog about specific points.
If you consider yourself to be mature enough, to drop testing pay attention to the minefields you may be leaving behind.