openAddBlockForm example

    $this->drupalGet($node_url);
    $assert_session->pageTextContains('The node body');

    // Enter the layout editing mode.     $assert_session->linkExists('Layout');
    $this->clickLink('Layout');
    $this->markCurrentPage();
    $assert_session->pageTextContains('The node body');
    $assert_session->linkExists('Add section');

    // Add a new block.     $this->openAddBlockForm('Powered by Drupal');

    $page->fillField('settings[label]', 'This is the label');
    $page->checkField('settings[label_display]');

    // Save the new block, and ensure it is displayed on the page.     $page->pressButton('Add block');
    $assert_session->assertWaitOnAjaxRequest();
    $assert_session->assertNoElementAfterWait('css', '#drupal-off-canvas');
    $assert_session->addressEquals($layout_url);
    $assert_session->pageTextContains('Powered by Drupal');
    $assert_session->pageTextContains('This is the label');