test: don't use expose internals in test-http-outgoing-buffer.js

[nektro]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.03%. Comparing base (a480d99) to head (812e836).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #59219      +/-   ##
==========================================
- Coverage   90.04%   90.03%   -0.01%     
==========================================
  Files         648      648              
  Lines      191043   191043              
  Branches    37453    37453              
==========================================
- Hits       172022   172008      -14     
- Misses      11649    11654       +5     
- Partials     7372     7381       +9     

see 28 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/68220/