Fix window.open loading

This is similar to https://github.com/servo/servo/issues/31973

When doing window.open the about:blank case should not result in any navigation and firing of load events.

Spec: https://html.spec.whatwg.org/multipage/#window-open-steps

In our current implementation, we do:

1. A synchronous load of a new BC. This always results in firing a load event.
2. We start a navigation-like workflow via ScriptMsg::ScriptNewAuxiliary

So, specwise, the about:blank case needs to be handled through the step at https://html.spec.whatwg.org/multipage/nav-history-apis.html#apis-for-creating-and-navigating-browsing-contexts-by-name:url-and-history-update-steps, which prevents the "otherwise, navigate" step below it.

In terms of fixing the implementation, I would say for "about:blank" we should create the "navigable"(browsing context sort of), but not do any navigation and not fire any load events. Specifically, the fact that ScriptThread::process_attach_layout always results in firing load events is wrong, and so is the fact that ScriptMsg::ScriptNewAuxiliary always results in a kind of navigation(I think). We would still need to somehow "tell" the constellation of the newly created BC, in case it is navigated later.

The relevant test is at /html/browsers/browsing-the-web/navigating-across-documents/initial-empty-document/load-pageshow-events-window-open.html. For some reason prior to https://github.com/servo/servo/pull/31505 we would pass one subtest, but since the test asserts unreached before a 50ms timeout, I think it was just luck(although not intermittent).

Start in components/script/dom/windowproxy.rs around the ScriptMsg::ScriptNewAuxiliary call and synchronous new browsing-context load. Trace ScriptThread::process_attach_layout and the constellation handoff, then run html/browsers/browsing-the-web/navigating-across-documents/initial-empty-document/load-pageshow-events-window-open.html. Done means about:blank creates the navigable without navigation or load events, while allowing later navigation.