os/signal: skip TestTerminalSignal if posix_openpt fails with EACCES

This happens in a chroot and so causes failures when packaging Go 1.10 for
Debian/Ubuntu.

Change-Id: I817038c237e584ce185b2168f8c7a10b9ef27b43
Reviewed-on: https://go-review.googlesource.com/90875


Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>