Use cookie as sole protocol for instant navigation testing (#89871)
The Instant Navigation Testing API already uses a cookie to persist lock
state across MPA navigations (page reloads, plain anchor links), since
window globals don't survive navigation. Rather than maintaining two
parallel mechanisms — a window global for SPA and a cookie for MPA —
this makes the cookie the single source of truth for all cases.
Playwright sets/clears the cookie directly via the browser context, and
the client reacts to changes through the CookieStore API's change event.
This removes the window.__EXPERIMENTAL_NEXT_TESTING__ global and the
page.evaluate() calls that went with it.
next.js
56adc02e - Use cookie as sole protocol for instant navigation testing (#89871)
