public function testEnsureStopped()
 {
     // this also test overlap between two periods
     $event = new StopwatchEvent(microtime(true) * 1000);
     $event->start();
     usleep(10000);
     $event->start();
     usleep(10000);
     $event->ensureStopped();
     $total = $event->getTotalTime();
     $this->assertTrue($total >= 27 && $total <= 40);
 }