[OVN][FT] Use MySQL backend for `Test*Maintenance` classes

This solution is similar to what was implemented in [1].

[1]https://review.opendev.org/q/topic:%22bug/2088423%22

Related-Bug: #2088423
Change-Id: I20a0f69ce51ac34a05c78792dc60cb9a924475c8

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

@@ -28,6 +28,7 @@ from neutron_lib import constants as n_const
 from neutron_lib import context as n_context
 from neutron_lib.exceptions import l3 as lib_l3_exc
 from oslo_utils import uuidutils
+from sqlalchemy.dialects.mysql import dialect as mysql_dialect
 
 from neutron.common.ovn import constants as ovn_const
 from neutron.common.ovn import utils
@@ -41,16 +42,20 @@ from neutron.tests.functional.services.logapi.drivers.ovn \
 
 from neutron.tests.unit.api import test_extensions
 from neutron.tests.unit.extensions import test_extraroute
+from neutron.tests.unit import testlib_api
+
 
 CFG_NEW_BURST = 50
 CFG_NEW_RATE = 150
 
 
-class _TestMaintenanceHelper(base.TestOVNFunctionalBase):
+class _TestMaintenanceHelper(testlib_api.MySQLTestCaseMixin,
+                             base.TestOVNFunctionalBase):
     """A helper class to keep the code more organized."""
 
     def setUp(self):
         super().setUp()
+        self.assertEqual(mysql_dialect.name, self.db.engine.dialect.name)
         self._ovn_client = self.mech_driver._ovn_client
         self._l3_ovn_client = self.l3_plugin._ovn_client
         ext_mgr = test_extraroute.ExtraRouteTestExtensionManager()