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

tests/__init__.py

@@ -0,0 +1,18 @@
+# Exposing all TestCases at the 'tests' module level
+# is required by the old (<=1.5) DjangoTestSuiteRunner.
+
+from .test_mailgun_backend import *
+from .test_mailgun_integration import *
+
+from .test_mandrill_backend import *
+from .test_mandrill_integration import *
+
+from .test_postmark_backend import *
+from .test_postmark_integration import *
+
+from .test_sendgrid_backend import *
+from .test_sendgrid_integration import *
+
+# Djrill leftovers:
+from .test_mandrill_djrill_features import *
+from .test_mandrill_webhook import *