internal/nettest: add TestableNetwork, NewLocalListener and NewLocalPacketListener
internal/nettest: add TestableNetwork, NewLocalListener and NewLocalPacketListener

Change-Id: I61475922625ce8dbeee4180807c3f021a502edf7
Reviewed-on: https://go-review.googlesource.com/34731
Run-TryBot: Mikio Hara <e2f4d2eec1d9a1f781345cdd7260d7d52e6468ae@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/net