Safari integration test flakiness

[jeffposnick]

There is some longstanding flakiness in our integration tests with Safari. With Safari 14.0.2, I can see the failures locally and on GitHub Actions with the should broadcast a message to all open window clients from workbox-broadcast-update, and reports all events for an external SW registration from workbox-window.

The commonality between those two tests is that they involve opening new tabs using webdriver, and relying on the service worker from the second tab to influence the behavior of the first.

From doing some local, manual testing, I don't believe that this is an issue outside of webdriver.

It would be great to get to the bottom of what's going on, as both tests pass in Firefox and Chrome.

C.f. #2321 which tracked a failure with the workbox-window test that also affected Firefox, but which I think is resolved. This failure seems to be something else.