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

---

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