Testing is covered but there are not extensive sections where test driven development is practiced (there's a short section when developing a library). Beginning Ruby covers a breadth of topics in a reasonable depth but not any specific things in intense depth. Design patterns, similarly, are not covered specifically. I consider these things to be useful once you reach the proficiency that Beginning Ruby will get a developer to, but then they can move on to Design Patterns in Ruby (which is an awesome book suitable for anyone who's finished BR!)
Author of Beginning Ruby (Apress): http://bit.ly/t31ag Editor of Ruby Inside and RubyFlow
A friend of mine once told me that Design Patterns exist mostly as a way to get around limitations in the language. One of the great things about Ruby (and Groovy, since I'm a zealot) is that you don't tend to have those limitations. There might be some performance improvements you can eek out, or more readable ways to write your code, but those tend to come from experience in my opinion.
I personally dont think we use design patterns in Java due to limitations in the language itself. For any (esp OO language) language it would be needed at some point while building a large enterprise application. Ruby being completely object oriented am sure people follow variety of design patterns in it as well.
Having said that, many design patterns applied for namesake and are not justified, but I think they are almost equally used in every object oriented language.
Its hard to defend this topic by example, but please share your thoughts and correct me if needed.