Unit testing

First recipe contribution, second commit includes license assignment. Also, since this is the first recipe for this section of the book, I also added a separate commit to include it in the appropriate section/chapter-level file.

Happy to make any edits you feel are necessary. The solution section is made a little long by showing the full output for running the example test, so I could see moving that to the discussion section if needed. Likewise my inclusion of (briefly) how to use Leiningen or Maven to run tests could be shortened/excluded if either build tool will be covered elsewhere in the book.