code: public function locate($configurationDir)
      {
          $conf = $configurationDir . '/phpunit.xml';
  
          if (file_exists($conf)) {
              return realpath($conf);
          }
  
          if (file_exists($conf . '.dist')) {
              return realpath($conf . '.dist');
          }
  
          throw new RuntimeException('Unable to locate phpunit.xml(.dist) file. This is required by Humbug.');
      }
docs: @param $configurationDir
  @return string