$cache_tags_header = $this->drupalGetHeader('X-Drupal-Cache-Tags');
$cache_tags = explode(' ', $cache_tags_header);
$this->assertTrue(in_array('aggregator_feed:' . $feed->id(), $cache_tags));
$cache_tags_header = $this->drupalGetHeader('X-Drupal-Cache-Tags');
$cache_tags = explode(' ', $cache_tags_header);
$this->assertTrue(in_array('aggregator_feed:' . $feed->id(), $cache_tags));