Clean up rbac_rule_validation unit tests

This patch simply makes the unit tests for rbac_rule_validation
cleaner, easier to read, and thus easier to maintain.

Change-Id: I9205909ff376ce94523b89499df415e9481a9e37
diff --git a/patrole_tempest_plugin/rbac_rule_validation.py b/patrole_tempest_plugin/rbac_rule_validation.py
index 69274b3..dd5b689 100644
--- a/patrole_tempest_plugin/rbac_rule_validation.py
+++ b/patrole_tempest_plugin/rbac_rule_validation.py
@@ -162,7 +162,7 @@
             except Exception as e:
                 exc_info = sys.exc_info()
                 error_details = exc_info[1].__str__()
-                msg = ("An unexpected exception has occurred during test: %s, "
+                msg = ("An unexpected exception has occurred during test: %s. "
                        "Exception was: %s"
                        % (test_func.__name__, error_details))
                 test_status = ('Error, %s' % (error_details))