Switch JS tests to WebDriverTestBase

The biggest change when switching to WebDriverTestBase is you can no longer inspect response status codes, see #2857562: Disabled methods to inspect headers for JavascriptTestBase tests

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!