class ConfigFormTestBase

abstract class ConfigFormTestBase extends KernelTestBase {
    
    /**
     * Form ID to use for testing.
     *
     * @var \Drupal\Core\Form\FormInterface
     */
    protected $form;
    
    /**
     * Values to use for testing.
     *
     * Contains details for form key, configuration object name, and config key.
     * Example:
     * @code
     *   array(
     *     'user_mail_cancel_confirm_body' => array(
     *       '#value' => $this->randomString(),
     *       '#config_name' => 'user.mail',
     *       '#config_key' => 'cancel_confirm.body',
     *     ),
     *   );
     * @endcode
     *
     * @var array
     */
    protected $values;
    
    /**
     * Submit the system_config_form ensure the configuration has expected values.
     */
    public function testConfigForm() {
        // Programmatically submit the given values.
        $values = [];
        foreach ($this->values as $form_key => $data) {
            $values[$form_key] = $data['#value'];
        }
        $form_state = (new FormState())->setValues($values);
        \Drupal::formBuilder()->submitForm($this->form, $form_state);
        // Check that the form returns an error when expected, and vice versa.
        $errors = $form_state->getErrors();
        $valid_form = empty($errors);
        $args = [
            '%values' => print_r($values, TRUE),
            '%errors' => $valid_form ? t('None') : implode(' ', $errors),
        ];
        $this->assertTrue($valid_form, new FormattableMarkup('Input values: %values<br/>Validation handler errors: %errors', $args));
        foreach ($this->values as $data) {
            $this->assertEquals($this->config($data['#config_name'])
                ->get($data['#config_key']), $data['#value']);
        }
    }

}