Fix scenario test_load_balancer_ipv6_CRUD

Change-Id: I590ec5f6b1dfebc454d2f16c9c5e2d691054d175
Related-PROD: PROD-33542
diff --git a/octavia_tempest_plugin/tests/test_base.py b/octavia_tempest_plugin/tests/test_base.py
index f8acc2b..5ad5236 100644
--- a/octavia_tempest_plugin/tests/test_base.py
+++ b/octavia_tempest_plugin/tests/test_base.py
@@ -307,6 +307,7 @@
                 cls.lb_member_vip_ipv6_net = {
                     'id': pub_ipv6_subnet[0]['network_id']}
             else:
+                cls.lb_member_vip_ipv6_net = cls.lb_member_vip_net
                 subnet_kwargs = {
                     'name': data_utils.rand_name("lb_member_vip_ipv6_subnet"),
                     'network_id': cls.lb_member_vip_net['id'],