net: make TestInterfaceAddrsWithNetsh more robust

TestInterfaceAddrsWithNetsh invokes Windows netsh command passing
it a particular interface name. This approach somehow does not work
on some computers (see issue for details). Change that to call netsh
without specifying any interface name. This provides output for all
interfaces available. So we can achieve same goal parsing this output.
Also makes test faster because we only need to invoke netsh once.

Fixes #14130.

Change-Id: I7911692ca64e372af1e1f9d6acb718c67071de67
Reviewed-on: https://go-review.googlesource.com/19441


Reviewed-by: Volker Dobler <dr.volker.dobler@gmail.com>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>