Implements \Drupal\views\Plugin\views\query\QueryPluginBase::buildOptionsForm().
Overrides PluginBase::buildOptionsForm
public function buildOptionsForm(&$form, &$form_state) {
parent::buildOptionsForm($form, $form_state);
$form['test_setting'] = array(
'#title' => t('Test setting'),
'#type' => 'textfield',
'#default_value' => $this->options['test_setting'],
);
}