do not test unconfigured resolvers

tests/test_dns_records.py

@@ -63,7 +63,7 @@ if os.path.isfile('../utils.py'):
     )
     for conf_resolver_key in conf_resolvers_keys:
         conf_resolver = conf.get(conf_resolver_key)
-        if conf_resolver not in resolvers:
+        if conf_resolver != '0' and conf_resolver not in resolvers:
             print('Resolver %s not configured on the system' % conf_resolver)
             all_ok = False