util/helper: add stderr param to ProcessWrapper

[ImHoppy]

Description

Add a stderr parameter to the ProcessWrapper, allowing the separation of stderr from the output. In my driver, I use this feature to ignore stderr by redirecting it to subprocess.DEVNULL

what do you use the feature for?
It allows calling ProcessWrapper without having stderr polluting the standard output.

how does labgrid benefit as a testing library from the feature?
This feature is used in my custom IpmiPowerDriver to check the status without having debug messages mixed.

how did you verify the feature works?
I tested the feature with a DUT in my labs.

if hardware is needed for the feature, which hardware is supported and which
hardware did you test with?
No specific hardware is required

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Checklist

• Documentation for the feature
• Tests for the feature
• The arguments and descriptions in doc/configuration.rst have been updated
• Add a section in doc/usage.rst to explain how to use the feature
• Add a section in doc/development.rst to explain how developers can use the feature
• PR has been tested locally
• Man pages have been regenerated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 45.4%. Comparing base (b666734) to head (0e84b68).
⚠️ Report is 104 commits behind head on master.

Additional details and impacted files

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  Coverage    45.4%   45.4%           
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  Files         177     177           
  Lines       13863   13865    +2     
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+ Hits         6300    6302    +2     
  Misses       7563    7563           

Flag	Coverage Δ
3.10	45.4% <100.0%> (+<0.1%)	⬆️
3.11	45.4% <100.0%> (+<0.1%)	⬆️
3.12	45.4% <100.0%> (+<0.1%)	⬆️
3.13	45.4% <100.0%> (+<0.1%)	⬆️
3.14	45.4% <100.0%> (+<0.1%)	⬆️

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[jluebbe]

Please add the description to the commit.

[Bastian-Krause]

Passing stderr=subprocess.DEVNULL or stderr=None is fine. But stderr=subprocess.PIPE would never drain the pipe nor close it. That's a problem. Maybe raise NotImplementedError if stderr is something else than None or subprocess.DEVNULL?

[Bastian-Krause]

Force-pushed: I've added the requested changes myself:

• use default similar to subprocess: subprocess.STDOUT means merging stdout and stderr, None means no capturing
• raise on unsupported stderr values
• adjusted the commit message accordingly

Attention: Since you're not using a real name, I've created #1940 to address that. Do not merge until #1940 is merged.