Tests the way service worker manager manages targets.

Target added: Main; type: page
Target added: service-worker-empty.js; type: service-worker
Target added: service-worker-empty.js; type: worker
Target removed: service-worker-empty.js; type: worker
Target removed: service-worker-empty.js; type: service-worker
Target added: service-worker-empty.js; type: service-worker