Switch JS tests to WebDriverTestBase

The biggest change when switching to WebDriverTestBase is you can no longer inspect response status codes, see [#2857562]

The suggestion in the change record is check for the existence of elements in the UI instead. This is already the case in most of \Drupal\Tests\search_api_autocomplete\FunctionalJavascript\IntegrationTest, so many of these can be removed without losing too much data.

However, a test like \Drupal\Tests\search_api_autocomplete\FunctionalJavascript\IntegrationTest::checkAdminAccess probably better off in a Functional test:

  /**
   * Verifies that admin pages are properly protected.
   */
  protected function checkAdminAccess() {
    // Make sure anonymous and non-admin users cannot access admin pages.
    $users = [
      'non-admin' => $this->normalUser,
      'anonymous' => NULL,
    ];
    $paths = [
      'index overview' => $this->getAdminPath(),
      'search edit form' => $this->getAdminPath('edit'),
      'search delete form' => $this->getAdminPath('delete'),
    ];
    foreach ($users as $user_type => $account) {
      $this->drupalLogout();
      if ($account) {
        $this->drupalLogin($account);
      }
      foreach ($paths as $label => $path) {
        $this->drupalGet($path);
        $status_code = $this->getSession()->getStatusCode();
        $this->assertEquals(403, $status_code, "The $label is accessible for $user_type users.");
      }
    }
    $this->drupalLogin($this->adminUser);
  }

Attached patch completes the conversion to WebDriverTestBase

Note that integration with Search API Pages requires the patch from #3063684: Search block form should define a base form ID to be committed.

The Drupal test infrastructure is still downloading the latest tagged release for drupal/search_api_page:

08:08:58 sudo -u www-data /usr/local/bin/composer require --no-interaction 'drupal/search_api_page:@dev' --prefer-stable --no-progress --no-suggest --working-dir /var/www/html
08:09:05 ./composer.json has been updated
08:09:05 > Drupal\Core\Composer\Composer::ensureComposerVersion
08:09:05 Loading composer repositories with package information
08:09:06 Updating dependencies (including require-dev)
08:09:15 Package operations: 1 install, 0 updates, 0 removals
08:09:15 - Installing drupal/search_api_page (1.0.0-beta2): Downloading (100%)

Attached patch updates the package requirement, so it downloads the dev-version instead.

I was using an outdated remote branch for 8.x-1.x. This patch should apply properly.

Looks good!