Always disable S3 under pytest (#3132)

Author: rhysyngsun

pytest.ini

@@ -29,6 +29,7 @@ env =
   MITX_ONLINE_EMAIL_BACKEND=django.core.mail.backends.locmem.EmailBackend
   MITX_ONLINE_NOTIFICATION_EMAIL_BACKEND=django.core.mail.backends.locmem.EmailBackend
   MITX_ONLINE_ENVIRONMENT=pytest
+  MITX_ONLINE_USE_S3=False
   MITXONLINE_DOCKER_BASE_URL=
   OPENEDX_API_BASE_URL=http://localhost:18000
   OPENEDX_API_CLIENT_ID=fake_client_id