BigPipePlaceholderTestCase example

    // used in the expectations.     $cacheability_depends_on_session_only = [
      'max-age' => 0,
      'contexts' => ['session.exists'],
    ];
    $cacheability_depends_on_session_and_nojs_cookie = [
      'max-age' => 0,
      'contexts' => ['session.exists', 'cookies:big_pipe_nojs'],
    ];

    // 1. Real-world example of HTML placeholder.     $status_messages = new BigPipePlaceholderTestCase(
      ['#type' => 'status_messages'],
      '<drupal-render-placeholder callback="Drupal\Core\Render\Element\StatusMessages::renderMessages" arguments="0" token="_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA"></drupal-render-placeholder>',
      [
        '#lazy_builder' => [
          'Drupal\Core\Render\Element\StatusMessages::renderMessages',
          [NULL],
        ],
      ]
    );
    $status_messages->bigPipePlaceholderId = 'callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&amp;args%5B0%5D&amp;token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA';
    $status_messages->bigPipePlaceholderRenderArray = [