net: deflake TestDialTimeout
net: deflake TestDialTimeout

Fixes issue 3867.
Fixes issue 3307.

R=bradfitz, dvyukov
CC=golang-dev
https://codereview.appspot.com/7735044

Committer: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang