PHP - Simpletest - How to test “included” classes

I'm trying to write test for changes to code for a Wordpress plugin.

<?php
require_once('.\simpletest\autorun.php');
require_once('.\wp-content\plugins\appointment-booking\includes.php'); 
// The line above causes [No runnable test cases in...] problem. 
// In my actual code, I have absolute path and I've made sure that the path is correct.

class test extends UnitTestCase 
{
 function test_pass()
 {
    $this->assertFalse(false);
 }
}
?>

Please help me understand why and how to fix.

I see this exact problem described here: PHP - Simpletest - How to test "included" classes, but I don't understand the problem, or how to fix. Thank you.