Add extension check for UpdateSubnetTest

subnet_allocation is required for following tests:
- test_update_allocation_pools
- test_update_allocation_pools_to_empty
- test_update_to_no_allocation_pools

Related-PROD: PRODX-5284
Change-Id: I066bd1f87e124293968afcc9487edba5582dfc51
(cherry picked from commit 8281cd77fb7dbe3608d169e777a31d342c35c0c9)
diff --git a/heat_tempest_plugin/tests/functional/test_create_update_neutron_subnet.py b/heat_tempest_plugin/tests/functional/test_create_update_neutron_subnet.py
index 9a10ba2..fab6e6e 100644
--- a/heat_tempest_plugin/tests/functional/test_create_update_neutron_subnet.py
+++ b/heat_tempest_plugin/tests/functional/test_create_update_neutron_subnet.py
@@ -10,6 +10,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+from tempest.common import utils
 from tempest.lib import decorators
 
 from heat_tempest_plugin.tests.functional import functional_base
@@ -44,6 +45,7 @@
         return output
 
     @decorators.idempotent_id('af43fc6d-58ba-4b5f-bd68-07b29f0a96bc')
+    @utils.requires_ext(extension='subnet_allocation', service='network')
     def test_update_allocation_pools(self):
         stack_identifier = self.stack_create(template=test_template)
         alloc_pools = self.get_outputs(stack_identifier, 'alloc_pools')
@@ -61,6 +63,7 @@
                          new_alloc_pools)
 
     @decorators.idempotent_id('2f1d3b04-2183-4e50-8aa3-fa0fbff5c861')
+    @utils.requires_ext(extension='subnet_allocation', service='network')
     def test_update_allocation_pools_to_empty(self):
         stack_identifier = self.stack_create(template=test_template)
         alloc_pools = self.get_outputs(stack_identifier, 'alloc_pools')
@@ -77,6 +80,7 @@
         self.assertEqual([], new_alloc_pools)
 
     @decorators.idempotent_id('6ec51150-6f83-4179-9faa-a3aee91ea150')
+    @utils.requires_ext(extension='subnet_allocation', service='network')
     def test_update_to_no_allocation_pools(self):
         stack_identifier = self.stack_create(template=test_template)
         alloc_pools = self.get_outputs(stack_identifier, 'alloc_pools')