Add tests with @cloudflare/vitest-pool-workers #22523

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.