net: tolerate permission errors in interface tests

net: tolerate permission errors in interface tests

On our linux-arm64 builders, we're getting permission errors despite
running as root. Detect those errors and skip the test.

For #61414.

Change-Id: I5d7c45789337bee3860b19335bbb9eb884c48986
Reviewed-on: https://go-review.googlesource.com/c/go/+/510737
Auto-Submit: Heschi Kreinick <heschi@google.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>