Add React Testing Library Course (#8742)

Co-authored-by: Eric Hellman <eric@hellman.net>

courses/free-courses-en.md

@@ -893,6 +893,7 @@
 * [React Patterns for web apps](https://www.patterns.dev/book/) - Lydia Hallie, Addy Osmani
 * [React Router 6 – Tutorial for Beginners](https://www.youtube.com/watch?v=59IXY5IDrBA) - John Smilga (freeCodeCamp)
 * [React State Management using Context API (useContext + useReducer Hooks = Magic)](https://www.youtube.com/watch?v=zxP4oGejqpU) - The Full Stack Junkie
+* [React Testing Library Crash Course](https://www.youtube.com/playlist?list=PL4cUxeGkcC9gm4_-5UsNmLqMosM-dzuvQ) - The Net Ninja
 * [React Tutorials](https://www.youtube.com/playlist?list=PLWKjhJtqVAbkArDMazoARtNz1aMwNWmvC) - freeCodeCamp
 * [ReactJS - Tutorial for Beginners](https://www.youtube.com/watch?v=Ke90Tje7VS0) - Mosh Hamedani (Programming with Mosh)
 * [ReactJS Basics](https://www.youtube.com/playlist?list=PLe30vg_FG4OSw8SIcLVci-jB_-W1ZkLYp) - Bitfumes