fix: test update for edx-drf-extensions 8.11.0

The method generate_unversioned_payload in
edx-drf-extensions now requires a user.id to
function, so that needed to be added to the mock
user.

ecommerce/tests/mixins.py

@@ -137,6 +137,7 @@ def generate_new_user_token(self, username, email, is_staff):
         # create a mock user, and not the actual user, because we want to confirm that
         #   the user is created during JWT authentication
         user = Mock()
+        user.id = 1
         user.username = username
         user.email = email
         user.is_staff = is_staff