Move tests out of app module

(Directory structure as suggested in
[Django testing docs][1].)

[1]: https://docs.djangoproject.com/en/1.9/topics/testing/advanced/#using-the-django-test-runner-to-test-reusable-applications

runtests.py

@@ -52,7 +52,7 @@ except ImportError:
 
 def runtests(*args):
     test_runner = TestRunner(verbosity=1)
-    test_labels = args if len(args) > 0 else [APP]
+    test_labels = args if len(args) > 0 else ['tests']
     failures = test_runner.run_tests(test_labels)
     sys.exit(failures)