Support MPA navigations in `instant()` (#89469)
Based on:
- https://github.com/vercel/next.js/pull/89465
---
The `instant()` testing API allows e2e tests to assert on prefetched UI
before dynamic data streams in. This extends it to handle full page
navigations: page reloads, plain anchor clicks, and browser
back/forward.
For client-side navigations, the API uses request headers to trigger
static shell rendering. For MPA navigations, headers aren't available,
so this uses a cookie-based approach instead. When `instant()` acquires
a lock, it sets a cookie that tells the server to render the static
shell. When the lock is released, the cookie is cleared and the page
reloads to fetch dynamic content.
On pages loaded with the test cookie, hydration is skipped entirely and
a minimal testing API is set up in the bootstrap script. This is similar
to how the `__nextppronly` debug parameter works (an internal experiment
that was never shipped). This feature is the productionized version of
that approach, designed for actual use in e2e tests.
New test cases:
- Static shell on page reload
- Static shell on plain anchor navigation
- Multiple MPA navigations within a single `instant()` scope
- Verifies navigations work normally after exiting `instant()` scope
