Fix test_def_link tests by adding missing mock to open() call

The mock was setup to only return the mocked data once, but the
 function is called twice under cfg_yaml.py, and it breaks at
 __yaml_load as there's no mock anymore.

tests/test_yamlcfg.py

@@ -90,6 +90,7 @@ profiles:
         '''.format(cfgstring)
 
         mock_open.side_effect = [
+                unittest.mock.mock_open(read_data=data).return_value,
                 unittest.mock.mock_open(read_data=data).return_value
                 ]
         mock_exists.return_value = True