Checks to see whether a block appears on the page.
\Drupal\block\Plugin\Core\Entity\Block $block: The block entity to find on the page.
protected function assertBlockAppears(Block $block) {
$result = $this
->findBlockInstance($block);
$this
->assertTrue(!empty($result), format_string('Ensure the block @id appears on the page', array(
'@id' => $block
->id(),
)));
}