public function testOAuthRefreshToken()
 {
     if (!isset($_SERVER['CB_OAUTH_ACCESS_TOKEN']) || !isset($_SERVER['CB_OAUTH_REFRESH_TOKEN'])) {
         $this->markTestSkipped('Environment variables CB_OAUTH_ACCESS_TOKEN and/or CB_OAUTH_REFRESH_TOKEN are missing');
     }
     $configuration = Configuration::oauth($_SERVER['CB_OAUTH_ACCESS_TOKEN'], $_SERVER['CB_OAUTH_REFRESH_TOKEN']);
     $configuration->setApiUrl(Configuration::SANDBOX_API_URL);
     $client = Client::create($configuration);
     try {
         $client->refreshAuthentication();
     } catch (InvalidRequestException $e) {
         $this->markTestSkipped('The OAuth token is invalid');
     }
 }