If you consider yourself to be mature enough, to drop testing pay attention to the minefields you may be leaving behind.
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.