net: default to 127.0.0.1, not localhost, in TestICMP
net: default to 127.0.0.1, not localhost, in TestICMP

If localhost resolves to ::1, the IPv4-only test fails.

R=rsc, r2
CC=golang-dev
http://codereview.appspot.com/4444084

webfd/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,rdp/rogerpack2005-golang,npe9/noahevans-go9,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,npe9/noahevans-go9,rdp/rogerpack2005-golang,scirelli/scirelli-go,mhennings/marcohennings-go,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,webfd/go-zh,npe9/noahevans-go9,sanjosh/sanjos100-tipc,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,mhennings/marcohennings-go,scirelli/scirelli-go,Triskite/willstone-goclone,bryanxu/go-zh,Triskite/willstone-goclone,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,d0f/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,rflanagan/reginaldflanagan-project1,rdp/rogerpack2005-golang,scirelli/scirelli-go,Triskite/willstone-goclone,bryanxu/go-zh,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,webfd/go-zh,bryanxu/go-zh,Triskite/willstone-goclone,webfd/go-zh,rdp/rogerpack2005-golang,mhennings/marcohennings-go,scirelli/scirelli-go,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,webfd/go-zh,npe9/noahevans-go9,npe9/noahevans-go9,webfd/go-zh,scirelli/scirelli-go,rdp/rogerpack2005-golang,npe9/noahevans-go9,rdp/rogerpack2005-golang,Triskite/willstone-goclone,d0f/go-zh,rflanagan/reginaldflanagan-project1,npe9/noahevans-go9,Triskite/willstone-goclone,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,scirelli/scirelli-go,npe9/noahevans-go9,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh