Add nested snat test

This test checks connectivity when ovn_router_indirect_snat is enabled.

This functionality was added for ML2/OVN in https://review.opendev.org/c/openstack/neutron/+/926495

Depends-On: https://review.opendev.org/c/openstack/neutron-tempest-plugin/+/940906

Resolves-Bug: OSPRH-13329

Change-Id: I2b31d1c123286bcbd69f932bda069aa3af9beb18

whitebox_neutron_tempest_plugin/tests/scenario/test_nested_routers.py

@@ -0,0 +1,47 @@
+# Copyright 2025 Red Hat, Inc.
+# All Rights Reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from neutron_tempest_plugin.scenario import test_floatingip
+from tempest import config
+from tempest.lib import decorators
+from whitebox_neutron_tempest_plugin.tests.scenario import base
+
+CONF = config.CONF
+WB_CONF = config.CONF.whitebox_neutron_plugin_options
+
+
+class DefaultSnatToExternalTest(
+    base.BaseTempestWhiteboxTestCase, test_floatingip.DefaultSnatToExternal
+):
+    @classmethod
+    def resource_setup(cls):
+        super(DefaultSnatToExternalTest, cls).resource_setup()
+        if cls.check_service_setting(
+            {"client": cls.master_node_client},
+            config_files=(
+                cls.neutron_conf,
+                "/etc/neutron/neutron.conf.d/02-neutron-custom.conf",
+            ),
+            section="ovn",
+            param="ovn_router_indirect_snat",
+            msg="Backend doesn't enable nested SNAT.",
+        ):
+            cls.feature_enabled_bool = True
+        else:
+            cls.feature_enabled_bool = False
+
+    @decorators.idempotent_id('97473428-e17c-4fe2-a389-b931dbfab6b2')
+    def test_nested_snat_external_ip(self):
+        self._test_nested_snat_external_ip(self.feature_enabled_bool)