assertFieldByName example

public function validateForm(array &$form, FormStateInterface $form_state) {}

  /** * Tests that values are returned. */
  public function testTextFormatElement() {
    /** @var \Drupal\Core\Form\FormBuilder $form_builder */
    $form_builder = $this->container->get('form_builder');
    $form = $form_builder->getForm($this);
    $output = $this->render($form);
    $this->setRawContent($output);
    $this->assertFieldByName('textformat[value]');
    $this->assertRaw('<h4>Full HTML</h4>');
    $this->assertRaw('<h4>Filtered HTML</h4>');
    $this->assertRaw('<h4>Test format</h4>');
    $this->assertNoPattern('|<h4[^>]*></h4>|', 'No empty H4 element found.');
  }

}