This is a way we can `spy()` the `adapter.confirmReceivingFIle()`. Since we have `mock_bluetooth.js` (https://github.com/mozilla-b2g/gaia/blob/master/apps/system/test/unit/mock_bluetooth.js#L22), I would like to suggest you to add `confirmReceivingFile()` method in this mock object. Someday, the API could be changed, and we will know the mock object needed to update. And what APIs we have used in this object.