Ajax callback for the 'drivertext' element of the validation form.
function ajax_forms_test_validation_form_callback($form, $form_state) {
drupal_set_message("ajax_forms_test_validation_form_callback invoked");
drupal_set_message(t("Callback: drivertext=%drivertext, spare_required_field=%spare_required_field", array(
'%drivertext' => $form_state['values']['drivertext'],
'%spare_required_field' => $form_state['values']['spare_required_field'],
)));
return '<div id="message_area">ajax_forms_test_validation_form_callback at ' . date('c') . '</div>';
}