Adds the 'JavaScript Testing Fundamentals' course (#4996)

* Adds the 'JavaScript Testing Fundamentals' course

Adds the free 'JavaScript Testing Fundamentals' course on JavaScript testing from https://vueschool.io

* Fix build issue.

* Fix alphabetization

* Migrates the JavaScript Testing Course to Vue.js 

Migrates the JavaScript Testing Course to Vue.js as the course uses Vue.js as a base for the testing course.

free-courses-en.md

@@ -405,6 +405,7 @@
 * [Internationalization with vue-i18n](https://vueschool.io/courses/internationalization-with-vue-i18n)
 * [Intro to Vue 2](https://www.vuemastery.com/courses/intro-to-vue-js/vue-instance)
 * [Intro to Vue 3](https://www.vuemastery.com/courses/intro-to-vue-3/intro-to-vue3)
+* [JavaScript Testing Fundamentals](https://vueschool.io/courses/javascript-testing-fundamentals)
 * [Nuxt.js Fundamentals](https://vueschool.io/courses/nuxtjs-fundamentals)
 * [Vue Router for Everyone](https://vueschool.io/courses/vue-router-for-everyone)
 * [Vue.js Components Fundamentals](https://vueschool.io/courses/vuejs-components-fundamentals)