Drain stdout and stderr concurrently

[jflan-dd]

Same motivation as #614, but for when stderr fills up first.

Can be manually verified using:

let shell = ShellExecutor()
let result = try shell.spawn("hexdump /dev/urandom | head -c 500000 1>&2", arguments: [])

[f-meloni]

Thank you!

[jflan-dd]

Thanks for the review and approval @f-meloni!

I don't have permission to merge the PR on my own. Could you merge it as well?