Concurrency bugs with with-redefs and deftest

pmap.clj

;; with-redefs
``` clojure
(defn print-it []
  (println "don't print this"))

(defn dont-print [x]
  (with-redefs [print-it (constantly nil)]
    (print-it)))

(defn -main []
  (doall (pmap dont-print (range 1000)))
  nil)
```

;;$ lein run
;;don't print this
;;don't print this
;;don't print this
;;don't print this

;; deftest/is
(deftest test-counter-parallel
  (doall (pmap (fn [x] (is true)) (range 1000))))

(deftest test-counter
  (doall (map (fn [x] (is true)) (range 1000))))

;;$ lein test :only pmap-bugs.core-test/test-counter-parallel
;;
;;lein test pmap-bugs.core-test
;;
;;Ran 1 tests containing 194 assertions.
;;0 failures, 0 errors.
;;$ lein test :only pmap-bugs.core-test/test-counter
;;
;;lein test pmap-bugs.core-test
;;
;;Ran 1 tests containing 1000 assertions.
;;0 failures, 0 errors.