custom.el

(defun clojure-test-filename ()
  (concat (projectile-project-root) 
          "test/" 
          (mapconcat #'identity
                     (butlast (split-string (cider-current-ns) "\\.")) "/")
          "/"
          (file-name-nondirectory (file-name-sans-extension (buffer-file-name)))
          "_test.clj"))

(defadvice projectile-toggle-between-implementation-and-test (around create-clojure-test-advice)
  "Visit new file if can't find test"
  (condition-case nil
      ad-do-it
    (error (find-file (clojure-test-filename)))))

(ad-activate 'projectile-toggle-between-implementation-and-test)