odoo.define('snippet_testimonial_options',function(require) {

    'use strict';

    var options = require('web_editor.snippets.options');

options.registry.snippet_testimonial_options=options.Class.extend({

on_focus: function(){

                          alert("On focus!")

                                    },

                        });

});

Hello, 

Sorry the doc for v9 theme seems to be not already updated.

Find below an example:

odoo.define('foo.bar', function (require) {
    'use strict';
    var website = require('website.website');
    var options = require('web_editor.snippets.options');
    options.registry.bar = s_options.Class.extend({
        start: function () {
            var self = this;
            this.$content = this.$target.find('.lorem');
        },
    });
});

In any case,s you can easily find many example in addons website (website/static/src/js/) and others modules (website_*)