Partial fix for Issue #3307617 by Grevil: Add installation tests and functionality tests

<?php

namespace Drupal\Tests\twig_real_content\Functional;

use Drupal\Tests\BrowserTestBase;

/**

 * This class provides methods specifically for testing something.

 *

 * @group twig_real_content

 */

class TwigRealContentFunctionalTest extends BrowserTestBase {

  /**

   * {@inheritdoc}

   */

  protected static $modules = [

    'node',

    'test_page_test',

    'twig_real_content',

  ];

  /**

   * A user with authenticated permissions.

   *

   * @var \Drupal\Core\Session\AccountInterface

   */

  protected $user;

  /**

   * A user with admin permissions.

   *

   * @var \Drupal\Core\Session\AccountInterface

   */

  protected $adminUser;

  /**

   * {@inheritdoc}

   */

  protected $defaultTheme = 'stark';

  /**

   * {@inheritdoc}

   */

  protected function setUp(): void {

    parent::setUp();

    $this->config('system.site')->set('page.front', '/test-page')->save();

    $this->user = $this->drupalCreateUser([]);

    $this->adminUser = $this->drupalCreateUser([]);

    $this->adminUser->addRole($this->createAdminRole('admin', 'admin'));

    $this->adminUser->save();

    $this->drupalLogin($this->adminUser);

  }

  /**

   * Tests if installing the module, won't break the site.

   */

  public function testInstallation() {

    $session = $this->assertSession();

    $this->drupalGet('<front>');

    // Ensure the status code is success:

    $session->statusCodeEquals(200);

    // Ensure the correct test page is loaded as front page:

    $session->pageTextContains('Test page text.');

  }

  /**

   * Tests if uninstalling the module, won't break the site.

   */

  public function testUninstallation() {

    // Go to uninstallation page an uninstall twig_real_content:

    $session = $this->assertSession();

    $page = $this->getSession()->getPage();

    $this->drupalGet('/admin/modules/uninstall');

    $session->statusCodeEquals(200);

    $page->checkField('edit-uninstall-twig-real-content');

    $page->pressButton('edit-submit');

    $session->statusCodeEquals(200);

    // Confirm uninstall:

    $page->pressButton('edit-submit');

    $session->statusCodeEquals(200);

    $session->pageTextContains('The selected modules have been uninstalled.');

    // Retest the frontpage:

    $this->drupalGet('<front>');

    // Ensure the status code is success:

    $session->statusCodeEquals(200);

    // Ensure the correct test page is loaded as front page:

    $session->pageTextContains('Test page text.');

  }

}