Fixes #17224.

Some systems have more than just "lo" in a fresh network namespace, due
to IPv6. Instead of testing for exactly 3 lines of output (implying 1
interface), just test to make sure that the unshare call resulted in
fewer interfaces than before. This should still verify that unshare did
something.

Change-Id: Iaf84c2b0e673fc207059d62e2f4dd7583a753419
Reviewed-on: https://go-review.googlesource.com/30372Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Jessica Frazelle <me@jessfraz.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>