test: fix stderr Buffer assertion in exec encoding test

[greenheadHQ]

Replace the duplicate stdout assertion with the missing stderr Buffer assertion. The introducing PR also aimed to improve stderr coverage.

Refs: #10919

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.00%. Comparing base (dc3b46f) to head (2c6d24a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #66008   +/-   ##
=======================================
  Coverage   89.99%   90.00%           
=======================================
  Files         785      785           
  Lines      269282   269282           
  Branches    51296    51288    -8     
=======================================
+ Hits       242348   242356    +8     
+ Misses      17445    17416   -29     
- Partials     9489     9510   +21     

see 28 files with indirect coverage changes

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