Unit test failing, Error: Call to undefined method CodeIgniter\AjaxControllerTest::co

Code:

<?php

namespace CodeIgniter;
use CodeIgniter\Test\CIUnitTestCase;

class AjaxControllerTest extends CIUnitTestCase
{

  public function testAjax()
  {
    $result = $this->controller(\App\Controllers\Ajax::class)->execute('getFormData');
    $this->assertTrue($result->isOK());
  }

}
?>

This is the source for my test case, I've followed the tutorial from the CI4 website. What am I doing wrong here? I've tried including the ControllerTestTrait with no luck.