Add a testdriver hook or similar to test URL bar prerendering

[domenic]

In whatwg/html#7533 we are discussing standardization for URL-bar-triggered prerendering.

Chromium engineers have written a lot of web platform tests for prerendering, but they are currently based on the speculation rules proposal. It would be ideal if we could run these tests as if the browser were triggering the prerender via the URL bar, which exercises a slightly different code path (probably a less-complicated one).

That would also make the tests reusable by other browsers, including browsers like present-day Safari and Edge which perform URL bar prerendering (but not speculation rules prerendering).