Properly clean up transfer requests

Change-Id: I6697c502875f281f0174a46a725d29fadb73c754
diff --git a/designate_tempest_plugin/tests/base.py b/designate_tempest_plugin/tests/base.py
index a44db45..a9a5e03 100644
--- a/designate_tempest_plugin/tests/base.py
+++ b/designate_tempest_plugin/tests/base.py
@@ -98,6 +98,10 @@
         with context:
             callable_(*args, **kwargs)
 
+    def transfer_request_delete(self, transfer_client, transfer_request_id):
+        return utils.call_and_ignore_notfound_exc(
+            transfer_client.delete_transfer_request, transfer_request_id)
+
     def wait_zone_delete(self, zone_client, zone_id, **kwargs):
         self._delete_zone(zone_client, zone_id, **kwargs)
         utils.call_until_true(self._check_zone_deleted,