test: avoid truncating watched worker deps

[inoway46]

This fixes a flaky sequential/test-watch-mode-worker failure where the nested CJS worker dependency case can log {} instead of sub-dep v1.

The test rewrites watched worker dependency files to trigger --watch restarts, but default writeFileSync() temporarily truncates the file. A restarted worker can read the dependency during that empty-file window.

This change keeps the write-based restart trigger, but uses r+ to overwrite without truncation.

Reported in: https://github.com/nodejs/reliability/blob/main/reports/2026-05-02.md

not ok 5323 sequential/test-watch-mode-worker
  ---
  Test failure: 'should watch changes to nested worker dependencies - cjs'
  AssertionError [ERR_ASSERTION]: Expected values to be strictly deep-equal:
    [
      'sub-dep v1',
      "Completed running '/home/iojs/node-tmp/.tmp.5322/2.js'. Waiting for file changes before restarting...",
      "Restarting '/home/iojs/node-tmp/.tmp.5322/2.js'",
  +   '{}',
  -   'sub-dep v1',
      "Completed running '/home/iojs/node-tmp/.tmp.5322/2.js'. Waiting for file changes before restarting..."
    ]

Testing:

$ ./tools/test.py sequential/test-watch-mode-worker

$ ./tools/test.py -j1 --repeat 20 sequential/test-watch-mode-worker

Refs: #62368

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.65%. Comparing base (13e90d0) to head (cfcd3e7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63083      +/-   ##
==========================================
- Coverage   91.49%   89.65%   -1.84%     
==========================================
  Files         360      712     +352     
  Lines      151674   220498   +68824     
  Branches    23919    42278   +18359     
==========================================
+ Hits       138772   197685   +58913     
- Misses      12625    14664    +2039     
- Partials      277     8149    +7872     

see 475 files with indirect coverage changes

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