Need better sid generation in UserBlocksUnitTests->testWhosOnlineBlock()

sid is primary key so mostly md5(microtime()) is the same and second insert failed with

'UserBlocksUnitTests', 'exception', 
'SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry ''3886465a51d1dfce61c06aa04f332ecd'' for key 1', 
'Uncaught exception', 'PDOStatement->execute()', 1691, 
'/home/drupaltesting/web/public/sites/drupaltesting.deekayen.net/files/checkout/includes/database/database.inc'

This happens in modules\user\user.test

    $this->insertSession();
    $this->insertSession();
...
  private function insertSession(array $fields = array()) {
    $fields += array(
      'uid' => 0,
      'sid' => md5(microtime()),
      'timestamp' => REQUEST_TIME,
    );