test: reduce iteration count in test-child-process-stdout-flush-exit

[aduh95]

This commit was added by @addaleax to fix the CI in #58124, opening it as a separate PR as the automation keeps removing it from the other PR. This should be landed manually so the commit authorship is credited to Anna and not myself.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/66751/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.16%. Comparing base (a822a1c) to head (3a5b89e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #58273      +/-   ##
==========================================
- Coverage   90.17%   90.16%   -0.01%     
==========================================
  Files         631      631              
  Lines      186652   186652              
  Branches    36660    36670      +10     
==========================================
- Hits       168312   168299      -13     
- Misses      11121    11127       +6     
- Partials     7219     7226       +7     

see 21 files with indirect coverage changes

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