syscall: skip test if unprivileged_userns_clone sysctl is missing

The original test (CL 166460) didn't check the existence of
/proc/sys/kernel/unprivileged_userns_clone and continue the test
if the file doesn't exist.

Fixes #32459

Change-Id: Iab4938252fcaded32b61e17edf68f966c2565582
Reviewed-on: https://go-review.googlesource.com/c/go/+/180877


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>