Ask HN: Being Concrete in Tests

I remember reading in a book that in tests, you generally want to be concrete, not abstract. For example, you'd like to do:

    assert(response.status == 401)
    assert(error == "param 'foo' is invalid")
    

instead of:

    assert(response.status == Http.StatusUnauthorized)
    assert(error == HttpError.InvalidParameter("foo"))

Can anyone point me to resources (books, posts, tweets, whatever) that better illustrate this and justify it?