Remove "unstable_test" decorator local implementation

Since [1] this decorator is in neutron-lib and used in this
repository. This local implementation can be removed.

[1] https://review.opendev.org/#/c/560102/

Trivial-Fix

Change-Id: I3c5f135fafcc380918905d8af164b71504441dc0
diff --git a/neutron_tempest_plugin/scenario/test_migration.py b/neutron_tempest_plugin/scenario/test_migration.py
index f4b918c..410c64e 100644
--- a/neutron_tempest_plugin/scenario/test_migration.py
+++ b/neutron_tempest_plugin/scenario/test_migration.py
@@ -17,6 +17,7 @@
 
 from neutron_lib.api.definitions import portbindings as pb
 from neutron_lib import constants as const
+from neutron_lib.utils import test
 from tempest.common import utils
 from tempest.lib import decorators
 import testtools
@@ -224,7 +225,7 @@
 
 class NetworkMigrationFromDVRHA(NetworkMigrationTestBase):
 
-    @common_utils.unstable_test("bug 1756301")
+    @test.unstable_test("bug 1756301")
     @decorators.idempotent_id('1be9b2e2-379c-40a4-a269-6687b81df691')
     @testtools.skipUnless(
         CONF.neutron_plugin_options.l3_agent_mode == 'dvr_snat',
@@ -233,7 +234,7 @@
         self._test_migration(before_dvr=True, before_ha=True,
                              after_dvr=False, after_ha=False)
 
-    @common_utils.unstable_test("bug 1756301")
+    @test.unstable_test("bug 1756301")
     @decorators.idempotent_id('55957267-4e84-4314-a2f7-7cd36a2df04b')
     @testtools.skipUnless(
         CONF.neutron_plugin_options.l3_agent_mode == 'dvr_snat',
@@ -242,7 +243,7 @@
         self._test_migration(before_dvr=True, before_ha=True,
                              after_dvr=False, after_ha=True)
 
-    @common_utils.unstable_test("bug 1756301")
+    @test.unstable_test("bug 1756301")
     @decorators.idempotent_id('d6bedff1-72be-4a9a-8ea2-dc037cd838e0')
     @testtools.skipUnless(
         CONF.neutron_plugin_options.l3_agent_mode == 'dvr_snat',