unix: add missing chtmpdir in TestPpoll on openbsd
unix: add missing chtmpdir in TestPpoll on openbsd

This ensures the the fifos are created in a temporary directory.

Change-Id: I744122207700e77a7a180d4f61336a93f3e5efca
Reviewed-on: https://go-review.googlesource.com/c/sys/+/214479
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Run-TryBot: Matt Layher <76528ff2e917253aa2211f68e4ca60b5b846dcf5@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Matt Layher <76528ff2e917253aa2211f68e4ca60b5b846dcf5@gmail.com>

multiplay/golang-sys,golang/sys,golang/sys,multiplay/golang-sys,multiplay/golang-sys,multiplay/golang-sys,golang/sys