Fix cache-clean test

[timmywil]

Changes are more easily viewed with ?w=1.

• The cache-clean test expected photos to be cached by the time it got a response,
  but the service worker is responding before the put to the cache has completed. Adding a short timeout before checking the cache fixes the issue, while still allowing the sw to respond immediately.
• This was frustrating because my code was right and the test kept failing. This should fix it for any future students.

[jakearchibald]

Fixed with d795679

Consuming the image as a blob is the bit that I was missing, that means the rest of my code runs once the image has been fully fetched. Added a 500ms delay just to be sure.

Thanks for spotting this!