This change makes use of the socktest package instead of the non-thread
safe variable syscall.SocketDisableIPv6 for simulating unreachable
external networks.

Also adds -ipv4 flag, -ipv6 flag already exists, as a control knob for
testing on each of IPv4-only, IPv6-only and dual IP stack kernels.

Fixes #7687.

Change-Id: I82002007fd526e8cf4de207f935e721df049a22f
Reviewed-on: https://go-review.googlesource.com/8390Reviewed-by: Ian Lance Taylor <iant@golang.org>