fix flawed test

This test's `sync` stub uses the wrong options.success api. The test is wrong though it does not produce any error. Yet I was adding custom features to my fork when this piece of code generated a bug. It will likely produce a bug in upstream backbone too if it is not fixed now.

The problem is `options.success` takes `(model, response, options)` when dealing with save, sync and co. However it takes `(response)` when in `Backbone.sync` and *this is not documented*.

I suggest adding an entry in the doc to avoid this kind of problems in the future.