net: disable Windows netsh tests when netsh won't run

On my Surface Pro X running the insider preview,
running "netsh help" from Powershell started from the task bar works.
But running "powershell" at a cmd.exe prompt and then running
"netsh help" produces missing DLL errors.
These aren't our fault, so just skip the netsh-based tests if this happens.

Change-Id: I13a17e01143d823d3b5242d827db056bd253e3e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/293849


Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>