Initial focus assumption for tests

https://github.com/web-platform-tests/wpt/commit/bef892c489 replaced window.open() + polling for opening testharness windows with the WebDriver "New Window" command, which doesn't run focusing steps. This inadvertently broke tests that implicitly relied on initial focus, such as:

• https://crrev.com/c/6341204
• https://wpt.fyi/results/navigation-api/focus-reset/change-focus-again-in-blur-during-intercept.html?label=master&label=experimental&aligned&view=interop&q=label%3Ainterop-2025-navigation
• web-platform-tests/wpt#56270

I think we should clarify and maybe document what (if any) initial focus target WPT guarantees. If we decide not to guarantee anything, each test will need to set up its own initial focus target.

As a point of comparison, the Marionette testharness implementation currently provides focus=True, which means "the opened window will be focused" (similar to the pre-bef892c489 WebDriver behavior).

Start with the linked WebDriver New Window change, the assumptions documentation at web-platform-tests.org/writing-tests/assumptions.html, and the Marionette testharness implementation referenced in the issue. Compare the affected focus-dependent tests and determine whether WPT guarantees an initial focus target or requires each test to establish one. Done means the policy is decided and clearly documented, with affected tests aligned to that decision.