Split Tests/UnitTests/Scripts/tests.ts into per-polyfill spec files

### Summary

`Tests/UnitTests/Scripts/tests.ts` has grown to ~1.5k lines spanning 15+ `describe` blocks across 10+ polyfills (XHR, WebSocket, Blob, File, FileReader, TextEncoder/Decoder, URL, Console, Scheduling, AbortController, napi prototype isolation, ...). Every new polyfill grows this single file further, which hurts readability, makes ownership/blame noisy, and increases merge-conflict surface.

### Proposal

Split the suite into one spec file per polyfill (e.g. `Scripts/tests/xhr.ts`, `blob.ts`, `file.ts`, `filereader.ts`, ...) with a small barrel/entrypoint that the test harness loads. Keep shared helpers (server bootstrap, fixtures) in a common module.

### Context

Raised in review of #169 (File / FileReader polyfill). Filing as a standalone follow-up so that PR's diff stays focused on the polyfill itself rather than a large test reorganization.