Implements hook_field_settings_form().
function field_test_field_settings_form($field, $instance, $has_data) {
$settings = $field['settings'];
$form['test_field_setting'] = array(
'#type' => 'textfield',
'#title' => t('Field test field setting'),
'#default_value' => $settings['test_field_setting'],
'#required' => FALSE,
'#description' => t('A dummy form element to simulate field setting.'),
);
return $form;
}