Some WPT tests for service worker take very long time in a test.
For example, 
- virtual/off-main-thread-fetch/external/wpt/service-workers/service-worker/register-link-element.https.html
- external/wpt/service-workers/service-worker/register-link-element.https.html
- external/wpt/service-workers/service-worker/registration.https.html

They check the behavior of service worker with several patterns.
They can fail with time-out, but finally pass in retries. It means the average time is close to 6000ms.

Can we split them or mark slow?