Tests debugging of HTML5 Workers

1. Open DevTools, Scripts Panel; Tick Debug on Workers sidebar.
2. Reload the page.
3. Start Worker
4. Observe 2 workers appear in the worker sidebar pane (including non-existent-worker.js)"
5. Observe worker-primes.js and primes.js appear in scripts drop-down box.
6. Assure primes are being logged to test console below.
7. Set a breakpoint on one of worker scripts, assure it's hit.
8. Try causing an error in worker, observe it's logged in DevTools console and in test console below. Cause Error
9. Stop Worker