test: mark stringbytes-external-exceed-max tests as flaky on AIX

[joyeecheung]

I doesn't look like anyone is going to fix them any time soon. Mark them as flaky as they've been flaking the CI pretty frequently (11+9+8+.... out of last 100 CI runs):

Refs: #60494
Refs: https://github.com/nodejs/reliability/blob/main/reports/2025-11-03.md

[joyeecheung]

cc @nodejs/platform-aix

[github-actions]

Fast-track has been requested by @joyeecheung. Please 👍 to approve.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.56%. Comparing base (684c3b3) to head (f8c77ef).
⚠️ Report is 40 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #60565      +/-   ##
==========================================
- Coverage   88.56%   88.56%   -0.01%     
==========================================
  Files         704      704              
  Lines      207858   207863       +5     
  Branches    40058    40046      -12     
==========================================
- Hits       184089   184087       -2     
+ Misses      15807    15800       -7     
- Partials     7962     7976      +14     

see 44 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/70039/