chore: mark test-process-exit-code-validation.js flaky (#34588)

## Summary

`node_compat::parallel::test-process-exit-code-validation.js`
intermittently
times out at the 20 s per-test cap on macOS CI runners
(e.g. [run
25936726631](https://github.com/denoland/deno/actions/runs/25936726631/job/76250878372),
`macos-15-intel`).

The test's structure makes this an inherent timing risk rather than a
bug:
its main body spawns `deno` 34 times via `spawnSync` — 17 input cases ×
2 modes (`process.exit(code)` and `process.exitCode = code`). On slow
macOS Intel runners the cumulative startup overhead can exceed the cap
even when each individual subprocess finishes quickly.

Mark the test `flaky` so the runner retries it up to 3 times (same
treatment we already give other timing-sensitive tests such as
`test-crypto-keygen-async-dsa.js` and
`test-stream-readable-unpipe-resume.js`).

Refs denoland/deno#34123
Closes denoland/orchid#341

## Test plan

- [ ] CI run of `test node_compat` (especially the macOS shards) stays
green

Co-authored-by: divybot <divybot@users.noreply.github.com>
Co-authored-by: Divy Srivastava <me@littledivy.com>