syscall: skip TestUseCgroupFD if cgroupfs mounted RO

syscall: skip TestUseCgroupFD if cgroupfs mounted RO

The skipping logic should also trigger if /sys/fs/cgroup is
mounted read-only too. This is how it is mounted on the
ppc64le/p10 containers today.

Fixes #57262

Change-Id: Idc0ab050052ebf5777ac09f9519215b437b0ee7c
Reviewed-on: https://go-review.googlesource.com/c/go/+/456835
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>