How do I add unit tests to a Leiningen project?

At the top level make a test/ directory, and then create some file, say mytests.clj. Here's a sample (caveat: I didn't actually compile this, but simplified an existing test):

(ns mytests
    (:use clojure.test))

(defn myfixture [block] 
    (do 
        (println "before test")
        (block)
        (println "after test")))

(use-fixtures :each myfixture)

(deftest mytest
    (is (= 2 (+ 1 1))))