mock.js

$.mockjax({
    url: "jsonpTest",
    type: "GET",
    response: function( settings ) {
        this.responseText = "{callback:\"" + settings.data.callback + "\"}";
        // or for custom callback
        this.responseText = "{jsonpOrWhatEverYouCallIt:\"" + settings.data.callback + "\"}";
    }
});

// Then in the test, JSON.parse the response and make sure callback's value contains jQuery or something like that (since jQuery's generated callback has jQuery in the name)