Simple plugin don’t display content

You need to call your custom file into your main plugin file. Without doing that, your custom file will not get called. Think of your main plugin file as your themes functions.php file and your custom file like a custom file that also holds functions

By default, WordPress will only read your functions.php, and not your custom functions file. For wordpress to know to load that functions inside the custom file, you need to tell wordpress that there is a custom file with functions it should load. Normally, in a theme, you would use include or require_once. Here we would do the same, except, your path will differ from themes

So you can try the following in your main plugin file

require_once(dirname(__FILE__) . '/testhook.php');