“Tests shouldn’t verify units of code. Instead they should verify units of behavior: something that is meaningful for the problem domain and ideally something that a business person can recognize as useful. The number of classes it takes to implement such a unit of behavior is irrelevant. The unit could span across multiple classes or only one class, or even take up just a tiny method. [...] A test should tell a story about the problem your code helps to share, and this story should be cohesive and meaningful to a non-programmer.”