stable ordering of test output

[Yuya Nishihara]

On Tue, 7 Mar 2017 17:56:58 +0100, Pierre-Yves David wrote:
> On the other hand, this is probably not so bundle2 specific. We have 
> some "select" logic to read stdout and stderr as soon as possible. This 
> is the main suspect as it is possible that this logic behave different 
> under linux and other unix (not too much effort have been put into it).

posix.poll() waits every type of operation no matter if fd is e.g. writable
or not. IIRC, this doesn't always work on FreeBSD since the underlying resource
of read/write ends might be shared in the kernel.

But I don't think this is the source of the unstable output.