test: unflake prefetch-fallback-static in deploy (#99366)
the ISR fallback tests were written in a way that only worked if the
first retry yielded a concrete prerender. this is enough in `next
start`, but in deploy, the ISR fallback retry does not always succeed in
one attempt. this PR adjusts the `act()` assertions to support multiple
retry attempts. this is not easy to express using usual `act()`
patterns, so we try the assertion, and retry it if it threw an error
that indicates that an expected response didn't show up (other errors
are rethrown)
