Hehe, no it's not about proofing anything. I already agreed that using `assertTrue` etc. seems preferable for simple cases.

The reason I'm using it here is that the message in case of a failure is much more descriptive for lists. `assertEquals()` only says something like "expected size 2 but got size 1" (leaving you guessing which element is missing), while `containsOnly()` says "missed element 'Foo' in ['Foo', 'Bar']".