Fix TestOVNMechanismDriver ipv6 tests

- test_update_subnet_dhcp_options_in_ovn_ipv6_not_change
- test_enable_subnet_dhcp_options_in_ovn_ipv6
This tests will fail if host where unit tests has ipv6 dns_servers
configured. This patch mocks get_system_dns_servers to avoid tests
to look at the host configuration.

Closes-Bug: #2056778
Change-Id: I2e703ab4b63c90d7a14f0dc41d37b0a98163bce0
(cherry picked from commit 2a196fe)
(cherry picked from commit a8bf8cd)

Author: averdagu

neutron/tests/unit/plugins/ml2/drivers/ovn/mech_driver/test_mech_driver.py

@@ -1515,9 +1515,10 @@ def test_enable_subnet_dhcp_options_in_ovn_ipv4(self, grm, gps):
         self.mech_driver.nb_ovn.set_lswitch_port.assert_has_calls(
             set_lsp_calls, any_order=True)
 
+    @mock.patch.object(ovn_utils, 'get_system_dns_resolvers')
     @mock.patch.object(db_base_plugin_v2.NeutronDbPluginV2, 'get_ports')
     @mock.patch.object(n_net, 'get_random_mac')
-    def test_enable_subnet_dhcp_options_in_ovn_ipv6(self, grm, gps):
+    def test_enable_subnet_dhcp_options_in_ovn_ipv6(self, grm, gps, gsd):
         grm.return_value = '01:02:03:04:05:06'
         gps.return_value = [
             {'id': 'port-id-1', 'device_owner': 'nova:compute'},
@@ -1530,6 +1531,7 @@ def test_enable_subnet_dhcp_options_in_ovn_ipv6(self, grm, gps):
                  {'opt_value': '10::34', 'ip_version': 6,
                    'opt_name': 'dns-server'}]},
             {'id': 'port-id-10', 'device_owner': 'network:foo'}]
+        gsd.return_value = []
         subnet = {'id': 'subnet-id', 'ip_version': 6, 'cidr': '10::0/64',
                   'gateway_ip': '10::1', 'enable_dhcp': True,
                   'ipv6_address_mode': 'dhcpv6-stateless',
@@ -1696,7 +1698,9 @@ def test_update_subnet_dhcp_options_in_ovn_ipv6(self):
         self.mech_driver.nb_ovn.add_dhcp_options.assert_called_once_with(
             subnet['id'], **new_options)
 
-    def test_update_subnet_dhcp_options_in_ovn_ipv6_not_change(self):
+    @mock.patch.object(ovn_utils, 'get_system_dns_resolvers')
+    def test_update_subnet_dhcp_options_in_ovn_ipv6_not_change(self, gsd):
+        gsd.return_value = []
         subnet = {'id': 'subnet-id', 'ip_version': 6, 'cidr': '10::0/64',
                   'gateway_ip': '10::1', 'enable_dhcp': True,
                   'ipv6_address_mode': 'dhcpv6-stateless',