test: avoid repeated writes in watch helper

[trivikr]

The shared watch helper was repeatedly writing the watched file in the
run({ watch: true }) update path until the second run completed. On slow CI,
that can trigger another watch restart while the previous rerun is still
active. The test runner then terminates the in-flight child process with
SIGTERM, and the captured output can include both a failed file-level subtest
and the next successful run.

This changes the update path to use performFileOperation(), matching the
helper’s rename/create paths. For the run() API path, that schedules a single
delayed write instead of rewriting until the second run completes. The helper
still waits for the next completed run before asserting output.

This also tightens run completion detection to count only root summary
duration_ms lines, instead of any duration_ms field in the TAP output.

Historical context:

• The original watch test used a single write after the first run.
• dd31255d4f changed the test to repeated writes as a flake workaround.
• 1a4ae462f8 slowed that interval to common.platformTimeout(1000).
• 1c7795e52e later removed repeated writes from the normal CLI watch path.
• ea1a240633b reintroduced repeated writes for the new run() API watch
  path when the monolithic watch test was split.

The repeated-write retry helped ensure the watcher observed a change, but
keeping it active until the whole second run completes creates a new race.

Refs: https://github.com/nodejs/reliability/blob/main/reports/2026-05-17.md#jstest-failure

---

Assisted-by: openai:gpt-5.5

[trivikr]

This is also expected to fix flakiness in test-runner/test-run-watch-without-file

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.05%. Comparing base (a9b0a83) to head (8f08bd1).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63386      +/-   ##
==========================================
- Coverage   90.06%   90.05%   -0.01%     
==========================================
  Files         714      714              
  Lines      225629   225648      +19     
  Branches    42702    42704       +2     
==========================================
- Hits       203211   203209       -2     
- Misses      14232    14233       +1     
- Partials     8186     8206      +20     

see 43 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/73535/