assertBigPipeResponseHeadersPresent example

$connection = Database::getConnection();
    $log_count = $connection->select('watchdog')->countQuery()->execute()->fetchField();

    // By not calling performMetaRefresh() here, we simulate JavaScript being     // enabled, because as far as the BigPipe module is concerned, JavaScript is     // enabled in the browser as long as the BigPipe no-JS cookie is *not* set.     // @see setUp()     // @see performMetaRefresh()
    $this->drupalGet(Url::fromRoute('big_pipe_test'));
    $this->assertBigPipeResponseHeadersPresent();
    $this->assertSession()->responseHeaderNotContains('X-Drupal-Cache-Tags', 'cache_tag_set_in_lazy_builder');

    $this->setCsrfTokenSeedInTestEnvironment();
    $cases = $this->getTestCases();
    $this->assertBigPipeNoJsPlaceholders([
      $cases['edge_case__invalid_html']->bigPipeNoJsPlaceholder     => $cases['edge_case__invalid_html']->embeddedHtmlResponse,
      $cases['html_attribute_value']->bigPipeNoJsPlaceholder        => $cases['html_attribute_value']->embeddedHtmlResponse,
      $cases['html_attribute_value_subset']->bigPipeNoJsPlaceholder => $cases['html_attribute_value_subset']->embeddedHtmlResponse,
    ]);
    $this->assertBigPipePlaceholders([
      $cases['html']->bigPipePlaceholderId                             => Json::encode($cases['html']->embeddedAjaxResponseCommands),