os: check for ErrNotExist instead of ErrExist in TestNonPollable

os: check for ErrNotExist instead of ErrExist in TestNonPollable

An apparent typo in CL 484837 caused the test to check for ErrExist
instead of ErrNotExist when opening /dev/net/tun for read. That causes
the test to fail on platforms where /dev/net/ton does not exist,
such as on the darwin-amd64-longtest builder.

Updates #59545.

Change-Id: I9402ce0dba11ab459674e8358ae9a8b97eabc8d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/486255
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Commit-Queue: Bryan Mills <bcmills@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>