RFC: Add new BiDi tests exclusively via WPT PRs, not via custom vendor infra, to ensure cross-vendor reviews

Hello,

I would like to ask that we *submit new BiDi tests exclusively via WPT PRs*,
instead of using our own infra (e.g. Bugzilla/Phrabricator for Mozilla,
Monorail/Gerrit for Google/Chromium).

This is to ensure we can have *cross-vendor reviews, which should increase
the quality and interoperability of new tests.*

If we add new tests via our own infra, then they *often come as a surprise
to other vendors*, and if there are any comments about them then it is
usually *a burden on these vendors to follow up* and ask the original
author to address the comments.

A few recent examples include:

- network.authRequired (https://phabricator.services.mozilla.com/D189517)
- network.addIntercept (https://phabricator.services.mozilla.com/D191237)
- browsingContext.contextDestroyed (
https://phabricator.services.mozilla.com/D189473)

You can find most of them with "git log --grep=bugzilla-url
webdriver/tests/bidi" in the WPT repo.


*Ideally, most of these should have been submitted via WPT PRs.*
"Trivial" changes are fine to be exempted from this request. Examples of
trivial changes:

- Remove "scrollIntoView" from "browsingContext.captureScreenshot" (
https://phabricator.services.mozilla.com/D192579)
- Extend timeout for an invalidation test file (
https://phabricator.services.mozilla.com/D188719)

Naturally the definition of "trivial" may vary amongst different people. *A
good rule of thumb is: new test additions, especially new test file
additions, should be considered non-trivial.*


Thank you,
-- 
All the best,
- Thiago