go/packages: replace darwin with netbsd in TestSizes
go/packages: replace darwin with netbsd in TestSizes

The darwin/386 port has been removed per golang.org/issue/37610.
TestSizes needs an operating system that has both amd64 and 386 ports,
so darwin no longer qualifies. NetBSD has had its 386 port restored
recently in golang/go#31726, so it can be used instead.

Fixes golang/go#38319.
For golang/go#37610.

Change-Id: I37ce6d86ca3ddad43e9294e0de66f36091cba54a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/227552
Reviewed-by: Benny Siegert <05f4aee36fd25d0de2a07789fd6e73398db492cc@gmail.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools