cakephp/tests/test_app/Plugin/TestPlugin/config/routes.php

<?php
use Cake\Core\Configure;

Configure::write('PluginTest.test_plugin.routes', 'loaded plugin routes');

if (isset($routes)) {
    $routes->get(
        '/test_plugin',
        ['controller' => 'TestPlugin', 'plugin' => 'TestPlugin', 'action' => 'index'],
        'test_plugin:index'
    );
}