code: public function back()
      {
          if (! $this->history->back()) {
              return false;
          }
          $content = $this->retry();
          if (! $content) {
              $this->history->forward();
          }
  
          return $content;
      }
docs: Equivalent to hitting the back button on the browser.
  The browser history is unchanged on failure.
  The page content is refetched as there is no concept of content caching in SimpleTest.
  
  @return bool     True if history entry and fetch succeeded