Hello Carlos

Your Problem

=============

•  right now you are using old js (odoo-8 js).

  
  //OLD js(Odoo-8 snippets structure)
  (function() {
      'use strict';
      var website = openerp.website;
      website.openerp_website = {};
  
      website.snippet.options.snippet_testimonial_options = website.snippet.Option.extend({
          on_focus: function() {
              alert("On focus!");
          }
      })
  })();

•       So you need to use new snippets js structure that i mention below. 
  

Solution
=============

follow odoo9 snippets js structure like below code, 

odoo.define('website.snippets.editor', function (require) {
    'use strict';
   
    var options = require('web_editor.snippets.options');
    var snippet_editor = require('web_editor.snippet.editor');
   
    options.registry.snippet_testimonial_options = options.registry.Class.extend({
        on_focus: function() {
            alert("On focus!");
        },
    });
});

I hope my answer is helpful.

If any query so comment please.