ServiceWorker: Stabilize multiple-register.html

To avoid flaky timeout failures, this change...
- stops using scopes in the inclusive relationship.
- reduces the number of concurrent registrations.
- removes duplicated test.


BUG=n/a

Review URL: https://codereview.chromium.org/580403002

git-svn-id: svn://svn.chromium.org/blink/trunk@182380 bbb929c8-8fbe-4397-9dbb-9b2b20218538