test: reduce watch mode restart flakiness

[trivikr]

Fixes intermittent failures in
test-watch-mode-restart-esm-loading-error.mjs.

The test now starts waiting for the watch restart before writing the file and
waits for a platform-scaled settling period before the next mutation. This
reduces races with filesystem watch event delivery and dependency reporting
from the watched child process on slower CI machines.

Refs: https://github.com/nodejs/reliability/blob/main/reports/2026-05-16.md#jstest-failure

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Assisted-by: openai:gpt-5.5

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.04%. Comparing base (550f195) to head (1a627f9).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63390      +/-   ##
==========================================
- Coverage   90.06%   90.04%   -0.02%     
==========================================
  Files         714      714              
  Lines      225648   225648              
  Branches    42711    42698      -13     
==========================================
- Hits       203237   203192      -45     
- Misses      14200    14257      +57     
+ Partials     8211     8199      -12     

see 54 files with indirect coverage changes

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