ipv6: don't fail test on IPv6 disabled kernel

Change-Id: Ib667928f91808640021ea9d61bc5477824729655
Reviewed-on: https://go-review.googlesource.com/33253
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed