TST: avoid capturing stderr in tests using subprocesses

[neutrinoceros]

I scratched my head a couple minutes as I tried to understand why these tests were failing locally against numpy 2. Turns out, the error was trivial and had nothing to do with numpy:

ModuleNotFoundError: No module named 'psutil'

but it originally was not visible in pytest logs, so I propose to let stderr propagate naturally in this context to help future debugging.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.55%. Comparing base (b962f56) to head (995b028).
Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #827   +/-   ##
=======================================
  Coverage   97.55%   97.55%           
=======================================
  Files           9        9           
  Lines        1392     1392           
=======================================
  Hits         1358     1358           
  Misses         34       34           

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