Fixes a test issue for Firebase App Hosting (#15069)

Author: bkendall

mmv1/products/firebaseapphosting/Domain.yaml

@@ -55,6 +55,9 @@ examples:
       service_act_id: '"tf-test-domain-m"'
   - name: firebase_app_hosting_domain_full
     primary_resource_id: example
+    ignore_read_extra:
+      # This isn't guaranteed to be set right away.
+      - custom_domain_status
     vars:
       backend_id: 'domain-full'
       service_act_id: 'sa-id'