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 <tobias.klauser@gmail.com>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>

unix/syscall_openbsd_test.go

@@ -12,6 +12,7 @@ import (
 )
 
 func TestPpoll(t *testing.T) {
+	defer chtmpdir(t)()
 	f, cleanup := mktmpfifo(t)
 	defer cleanup()