/** @var \Drupal\Core\Utility\LinkGenerator $link_generator */
$link_generator = $this->container->get('link_generator');
-
$generated_url = Url::fromRoute('user.register', [], ['absolute' => TRUE])->toString(TRUE)->getGeneratedUrl();
$expected = [
'link via the linkgenerator: ' . $link_generator->generate('register', new Url('user.register', [], ['absolute' => TRUE])),