[FT] Make explicit the "publish" call check in "test_port_forwarding"

The functional test "TestMaintenance.test_port_forwarding" is checking
the "registry.publish" call after calling
"delete_floatingip_port_forwarding". This patch makes this check
explicit for the expected call.

Closes-Bug: #2033387
Change-Id: I95074a0fe071e3ed8bf58c9210d63a2f30ee68a0
(cherry picked from commit d9eb044)

neutron/tests/functional/plugins/ml2/drivers/ovn/mech_driver/ovsdb/test_maintenance.py

@@ -939,7 +939,9 @@ def _verify_lb(test, protocol, vip_ext_port, vip_int_port):
             m_publish.reset_mock()
             self.pf_plugin.delete_floatingip_port_forwarding(
                 self.context, pf_obj['id'], fip_id)
-            m_publish.assert_called_once()
+            call = mock.call('port_forwarding', 'after_delete', self.pf_plugin,
+                             payload=mock.ANY)
+            m_publish.assert_has_calls([call])
 
             # Assert load balancer for port forwarding is stale
             _verify_lb(self, 'udp', 5353, 53)