I think your test case should include all the situations, such as 
```
var api = xwalk.experimental.system
api.getCPUInfo().then( function(info) {...}, function(error) {...}); // have the whole two callback functions as the arguments.
api.getCPUInfo().then(function(info), function(error))).then(function(info), function(error)); // test the embeded 'then'
```