Remove usage of deprecated functions of WebTestBase

Meta issue: #2205673: [meta] Remove all @deprecated functions marked "remove before 8.0"

The following functions in the Drupal\simpletest\WebTestBase are deprecated and should be removed:

  /**
   * Sets the raw HTML content.
   *
   * @deprecated 8.x
   *   Use setRawContent().
   */
  protected function drupalSetContent($content) {
    $this->setRawContent($content);
  }

  /**
   * Sets the value of drupalSettings for the currently-loaded page.
   *
   * @deprecated 8.x
   *   Use setDrupalSettings().
   */
  protected function drupalSetSettings($settings) {
    $this->setDrupalSettings($settings);
  }

  /**
   * Gets the current raw HTML of requested page.
   *
   * @deprecated 8.x
   *   Use getRawContent().
   */
  protected function drupalGetContent() {
    return $this->getRawContent();
  }

  /**
   * Gets the value of drupalSettings for the currently-loaded page.
   *
   * @deprecated 8.x
   *   Use getDrupalSettings().
   */
  protected function drupalGetSettings() {
    return $this->getDrupalSettings();
  }