chore: use os.Stdout instead of t.Output()

[zirain]

xref: https://github.com/envoyproxy/gateway/actions/runs/24942232519/job/73037758485

Note: In production, certGen is called with cmd.OutOrStdout() which resolves to os.Stdout
in most cases, so using os.Stdout in tests mirrors real-world behavior.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.36%. Comparing base (7811d86) to head (f991936).
⚠️ Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8860      +/-   ##
==========================================
- Coverage   74.37%   74.36%   -0.02%     
==========================================
  Files         246      246              
  Lines       39287    39287              
==========================================
- Hits        29218    29214       -4     
- Misses       8040     8044       +4     
  Partials     2029     2029              

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