I'm not sure this example is 100%. This is what I used to get it working:

(ns com.puppetlabs.test.foo
  (:require [clojure.tools.logging :as log])
  (:use clojure.test
        [com.puppetlabs.puppetdb.testutils :only [with-log-output num-logs-matching]]))

(deftest foo?
  (testing "should work"
    (with-log-output logs
      (log/info "hello there")
      (is (= 1 (num-logs-matching #"hello there" @logs))))))