Two main sources of flakiness addressed:
1) we should dispose the lifecycle watcher after we waited for the navigation response (bad API? we need to refactor but I think it'd be valuable to stabilize tests first without too many changes).
2) we should wait for the navigation request's response if there is a navigation request in the watcher.
Closes#8644
* The testing tsconfig.json inherits from the base TS config.
* A lot of type assertions have been inserted...a lot.
* All testing utilities have migrated to TS.
* text-diff is being replaced with diff for TS compatibility.
* ProtocolError has been added to PuppeteerErrors and PuppeteerErrors is no longer a record (it's been frozen).
* Fixes a small bug where null was an allowable media type in emulation (should be undefined).
