nettest: use of internal/nettest package

Change-Id: I468795cc9ab4d2350d20427e62676df5d89b2c3b
Reviewed-on: https://go-review.googlesource.com/34733
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