testrunner: stream output from subprocesses

[ereslibre]

Why is this PR needed?

testrunner: stream output from subprocesses

Fixes: #680

Merge restrictions

(Please do not edit this)

We are in v4-maintenance phase, so we will restrict what can be merged to prevent unexpected surprises:

What can be merged (merge criteria):
    2 approvals:
        1 developer: code is fine
        1 QA: QA is fine
    there is a PR for updating documentation (or a statement that this is not needed)

[dannysauer]

I'm gonna go ahead and approve this despite my comment about removing stdbuff. If no one else looks at it and has an opinion, then it's fine to remove it for now and maybe add it back in later if we decide to change the read mechanism to handle those status-generating commands.

[dannysauer]

Ugly as it is, I think we might like to leave the stdbuf commands. There's some desire (SUSE/avant-garde#670) to have tests running within Jenkins or similar generate synchronous output. Imagine something like wget which prints progress output before a newline happens, for example. That, of course, will also require a tweak to how the read_fd threads read from the pipe, but we can deal with that later. This gets us 90% there.

[ereslibre]

I would like to only keep things we really need, otherwise we can just keep certain changes because they were there before without a real use. Honestly, if we need to have a complete unbuffered output let's do that in the future, in my opinion.

[pablochacin]

LGTM