Pin prefetch behavior in the modes without Partial Prefetching
Add tests for prefetch behavior that the upcoming prefetch cache changes
must keep, in the modes that don't use Partial Prefetching:
- A prefetch={true} link fetches only the dynamic data that a default
prefetch of the same URL left out, and a default link doesn't
re-request what a prefetch={true} link already fetched.
- Back/forward navigations are served from the back/forward cache
without requests.
- A loading-boundary prefetch counts as complete for the segments above
the boundary, so the navigation requests only the page below it.
force-stale no longer pins `cacheComponents`, so it runs both with and
without Cache Components.
