new_contents: from django.test import TestCase, override_settings
  
  from chatwork.backends import DummyBackend, UrllibBackend, get_backend
  
  
  class DummyBackendTests(TestCase):
      def test_return_empty(self):
          backend = DummyBackend()
          self.assertEqual(backend.get('/me'), {})
  
  class GetBackendTests(TestCase):
      def test_default_backend(self):
          with override_settings(DEBUG=True):
              self.assertEqual(get_backend(), DummyBackend)
          with override_settings(DEBUG=False):
              self.assertEqual(get_backend(), UrllibBackend)
  
subject: add tests for backends