✅(backend) fix failing test on refund an order

Fix a failing flaky unit test on refund an order when its state is canceled.
Once a refund is requested an email should be sent to notify the user.
The content email based on mjml templates and the footer part use a variable to fill the catalog base url.
None as default, we override it with the override_settings before function definition to check its value.

Author: Nicolas CAN

src/backend/joanie/tests/core/api/admin/orders/test_refund.py

@@ -156,6 +156,7 @@ def test_api_admin_orders_refund_an_order_not_possible_if_no_installment_is_paid
 
     @override_settings(
         JOANIE_CATALOG_NAME="Test Catalog",
+        JOANIE_CATALOG_BASE_URL="https://richie.education",
         DEFAULT_CURRENCY="EUR",
         JOANIE_PAYMENT_SCHEDULE_LIMITS={
             100: (20, 30, 30, 20),
@@ -327,7 +328,7 @@ def test_api_admin_orders_refund_an_order(self):
             f"Withdrawn on {order.payment_schedule[3]['due_date'].strftime('%m/%d/%Y')}",
             "Canceled",
             "Total €100.00",
-            f"This mail has been sent to {order.owner.email} by Test Catalog [None]",
+            f"This mail has been sent to {order.owner.email} by Test Catalog [https://richie.education]",
         ]
 
         self.assertEqual(