What do you mean by the term "JBoss" in your question? Do you mean JBoss Application Server? The book JBoss in Action? Or the JBoss development team?
As far as the book is concerned, it is not an application development book - the focus of the book is JBoss AS configuration. We present only the simplest of application examples (hello world or perhaps one step beyond that) and then only to provide complete configuration example.
JBoss AS is a framework for deploying and running applications, so the application server doesn't really provide support for the methodology with which you write your code. But several libraries and development frameworks (Seam comes to mind) the Red Hat / JBoss makes provide hooks for unit testing and creating Test Doubles. Also, in the last few years, the JBoss code itself has become much more tested / testable.