PushManager/PushSubscription requires a service worker registration

[queengooborg]

Both the PushManager and PushSubscription API require a service worker registration, as most of the properties are not available on them. However, to do so, we either a) need to register a new service worker in that moment, or b) use the existing service worker registration when running service worker-scoped tests, and overhaul the code to run service workers first and make sure we always load a service worker if we're able to.