test: fix watch cwd argv test flake

[trivikr]

This fixes a flaky watch mode test by keeping the marker file outside the
watched cwd.

The test runs run({ watch: true, cwd, isolation: 'none' }) without explicit
files, so the runner watches the whole cwd. On Linux, writing the marker file
inside that cwd can trigger a watch restart before the test aborts the stream,
causing a second test:pass event.

The marker is now written in the parent tmpdir while the test file lives in a
dedicated watched subdirectory.

Refs: https://github.com/nodejs/reliability/issues?q=%22test-run-watch-cwd-isolation-none-argv%22

Example

not ok 5145 test-runner/test-run-watch-cwd-isolation-none-argv
  ---
  duration_ms: 866.79500
  severity: fail
  exitcode: 1
  stack: |-
    Mismatched noop function calls. Expected exactly 1, actual 2.
        at Module.mustCall (/home/iojs/build/workspace/node-test-commit-linuxone/test/common/index.js:468:10)
        at file:///home/iojs/build/workspace/node-test-commit-linuxone/test/test-runner/test-run-watch-cwd-isolation-none-argv.mjs:39:31
        at ModuleJob.run (node:internal/modules/esm/module_job:447:25)
        at async node:internal/modules/esm/loader:646:26
        at async asyncRunEntryPointWithESMLoader (node:internal/modules/run_main:101:5)

---

Assisted-by: openai:gpt-5.5

[trivikr]

✅ stress test: https://ci.nodejs.org/job/node-stress-single-test/768/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/74482/