Provide necessary debug infromation in non-verbose test runs.

[furtib]

Why:
I want to be able to get enough information without having to scroll through pages of logs.
By default, pytest hides the current logs. I need to pass --log-cli-level=DEBUG to each run, to see them.

What:

• Add stderr as message for process asserts

Addresses:
none

[Szelethus]

I get the point, I'm just wondering if this is the way to do it -- will we always remember to add stderr as a third parameter? Can we just unconditionally print stderr from run_command()? Pytest actually handles output capturing rather elegantly. Though I'm not sure how much it'd litter the output for @nettle who IIRC doesn't use pytest (or not in every environment he tests in).

Still, the change as it stands is an improvement, once we put the effort into it, I suppose there is no reason not to accept it.

[furtib]

The idea behind #176 is to show the test author that they need to pass stderr as a 3rd parameter.
It doesn't feel like the best solution, but I have no better idea on how to improve it.