fix(api): ensure test uses locmem email backend

Author: nils-wisiol

api/desecapi/tests/test_mail_backends.py

@@ -18,7 +18,7 @@
     },
 )
 class MultiLaneEmailBackendTestCase(TestCase):
-    test_backend = settings.EMAIL_BACKEND
+    test_backend = 'django.core.mail.backends.locmem.EmailBackend'
 
     def test_lanes(self):
         debug_params = {"foo": "bar"}