[testdriver] Add testdriver extension for prefetch event in speculation module

[srwei]

Spec: https://wicg.github.io/nav-speculation/prefetch.html#automated-testing
chromium-bidi implementation: GoogleChromeLabs/chromium-bidi@66d3585

Adding testdriver extension for the speculation module that is mainly to add the prefetch_status_updated event. No commands.

This event will replace the current timeout mechanism for speculation rules prefetch in the wpt prefetch corpus: https://github.com/web-platform-tests/wpt/tree/master/speculation-rules/prefetch

Follow up CLs will be raised to update the above tests to use the new event handling.

@sadym-chromium would you be able to help with rollover for chromium-bidi implementation?

[sadym-chromium]

Please add wdspec tests as well. Actually, I'd prefer to have wdspec tests first in a separate PR, but it's up to you.

[srwei]

Please add wdspec tests as well. Actually, I'd prefer to have wdspec tests first in a separate PR, but it's up to you.

Oh I thought webdriver/tests/bidi/external/speculation/prefetch_status_updated/ tests were the wdpsec tests

[sadym-chromium]

Please add wdspec tests as well. Actually, I'd prefer to have wdspec tests first in a separate PR, but it's up to you.

Oh I thought webdriver/tests/bidi/external/speculation/prefetch_status_updated/ tests were the wdpsec tests

My bad, I missed it. Sure.

[sadym-chromium]

hm. BTW why do you need the global subscription? The events are emitted to the specific browsing context, so you could safely target them.

[srwei]

Initially was seeing how it would fit with the existing speculation rules prefetch wpt tests, but I was able to confirm we can utilize the new spawned window when subscribing to the event. I have changed it back to the normal browsing context subscription.