Add tests with @cloudflare/vitest-pool-workers

### Description

For now our `cloudflare-integration-tests` do run with `wrangler` directly, which is not a bad thing overall. However, wrangler runs with ESBuild inside and does tree-shaking - when running tests within vitest, then there is no tree-shaking and it would include everything. In #22519 there was a case where `worker_threads` were included, which weren't available in an older compatibility version. To not break older compatibility versions by accident tests with `@cloudflare/vitest-pool-workers` would be required to prevent such things.

Start by locating the cloudflare-integration-tests and reviewing how they currently run through wrangler. Read #22519 for the worker_threads compatibility case, then determine how @cloudflare/vitest-pool-workers should run equivalent tests; done means the tests cover this regression without breaking older compatibility versions.

Written by the indexing model from the issue text.