Installing Race Condition Test

[azizhk]

Prior to filing a PR, please: Based on discussion in #2209 #2216 This is a test case trying to recreate a race condition where the service worker is in "installing" state when the user/developer calls wb.register().

Workbox does not add listeners to pre-existing registration.installing service worker but only when updatefound event is triggered.

R: @jeffposnick @philipwalton

[azizhk]

I hope it's clear that my aim is not to defer "installing" but actually to programmatically create a condition where there exists a registration.installing SW when register is called.