Pass if a link containing a given href (part) is found.
$href: The full or partial value of the 'href' attribute of the anchor tag.
$index: Link position counting from zero.
$message: Message to display.
$group: The group this message belongs to, defaults to 'Other'.
TRUE if the assertion succeeded, FALSE otherwise.
protected function assertLinkByHref($href, $index = 0, $message = '', $group = 'Other') {
  $links = $this
    ->xpath('//a[contains(@href, :href)]', array(
    ':href' => $href,
  ));
  $message = $message ? $message : t('Link containing href %href found.', array(
    '%href' => $href,
  ));
  return $this
    ->assert(isset($links[$index]), $message, $group);
}