[test] Do not use sandboxes for hydration error tests (#83398)
The sandboxes used in the `Error overlay for hydration errors in App
router` test suite frequently lead to timeout errors in CI
([DD](https://app.datadoghq.com/ci/test/runs?query=test_level%3Atest%20%40git.repository.id%3A%22github.com%2Fvercel%2Fnext.js%22%20%40test.type%3A%22nextjs%22%20%40test.status%3A%22fail%22%20%40git.branch%3Acanary%20%40test.suite%3A%22Error%20overlay%20for%20hydration%20errors%20in%20App%20router%22&agg_m=count&agg_m_source=base&agg_t=count¤tTab=overview&eventStack=&fromUser=false&index=citest&start=1755633071704&end=1756929071704&paused=false))
and also make it unnecessarily complicated to run the scenarios locally
with `pnpm next dev`. I also have a suspicion that the sandboxes are not
properly cleaning up after themselves, leading to test interference.
By saving the inlined layouts and pages to actual files, we can avoid
the issues caused by sandboxes and make the tests faster, more reliable,
and easier to debug.
