Tests: Persistence test mixin fix

* The _get_connection method of PersistenceTestMixin
  should take `self` as the first argument.

Change-Id: Ibaaa0d54b408d5e37c6349c63becef919d02fac8

taskflow/tests/unit/persistence/base.py

@@ -25,7 +25,7 @@ from taskflow.utils import misc
 
 
 class PersistenceTestMixin(object):
-    def _get_connection():
+    def _get_connection(self):
         raise NotImplementedError()
 
     def test_logbook_save_retrieve(self):