Proof of Concept, catch test output

test.clj

(defonce ^:private log-buffer (atom nil))

(defn get-log-message []
  (let [message (first @log-buffer)]
    (swap! log-buffer #(vec (rest %)))
    message))

(defn catch-logging [f]
  (reset! log-buffer nil)
  (let [printer (java.io.PrintStream.
                  (proxy [java.io.ByteArrayOutputStream] []
                    (flush []
                      ; deal with reflection in proxy-super
                      (let [^java.io.ByteArrayOutputStream this this]
                        (proxy-super flush)
                        (let [message (.trim (.toString this))]
                          (proxy-super reset)
                          (if (> (.length message) 0)
                            (swap! log-buffer conj message))))))
                  true)
        orig-out System/out]
    (System/setOut printer)
    (try
      (f)
      (finally
        (System/setOut orig-out)))))

(test/use-fixtures :each catch-logging)

(deftest foo-test
  (log/infof "Hello world")
  (is (re-find #"^\d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2}\.\d{3} \+\d{4} \[[^\]]*\] INFO  \[lokit.core-test\] - Hello world$" (get-log-message))))