net: add IPConn through Conn test
net: add IPConn through Conn test

Also refactors mock ICMP stuff.

R=dave, rsc
CC=golang-dev
https://codereview.appspot.com/7325043

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