To test changes in mytestcode.js, all you need to do is re-execute CodeRunner every time you make an update to the file. No need for cutting-and-pasting.
Instead of cutting-and-pasting the entire mytestcode.js into the CodeRunner’s textbox, just use the code above and make your changes directly to the mytestcode.js file on your server.
Furthermore, you don’t need to put all your code in a single file. You can break them up into multiple files and call out the include function for each.
With this technique, you can even run your entire OpenSocial application in CoderRunner!