fix unit test
2018-09-28 12:11:25,592 [ERROR ] 'DarkRefsCopy' object has no attribute 'mutex' tomwer.core.darkref.darkrefs ERROR:tomwer.core.darkref.darkrefs:'DarkRefsCopy' object has no attribute 'mutex'
2018-09-28 12:11:25,592 [ERROR ] 'DarkRefsCopy' object has no attribute 'mutex' tomwer.core.darkref.darkrefs ERROR:tomwer.core.darkref.darkrefs:'DarkRefsCopy' object has no attribute 'mutex'