Pass if a link with the specified label is not found.
$label: Text between the anchor tags.
$message: Message to display.
$group: The group this message belongs to, defaults to 'Other'.
TRUE if the assertion succeeded, FALSE otherwise.
protected function assertNoLink($label, $message = '', $group = 'Other') {
  $links = $this
    ->xpath('//a[normalize-space(text())=:label]', array(
    ':label' => $label,
  ));
  $message = $message ? $message : t('Link with label %label not found.', array(
    '%label' => $label,
  ));
  return $this
    ->assert(empty($links), $message, $group);
}