net: make TestDialTimeoutFDLeak a bit robust
net: make TestDialTimeoutFDLeak a bit robust

TestDialTimeoutFDLeak will fail when system state somaxconn is
greater than expected fixed value.

Fixes issue 4384 (again).

R=fullung, dave, rsc
CC=golang-dev
https://codereview.appspot.com/6873069

webfd/go-zh,scirelli/scirelli-go,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,d0f/go-zh,d0f/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,mhennings/marcohennings-go,mhennings/marcohennings-go,rdp/rogerpack2005-golang,webfd/go-zh,rdp/rogerpack2005-golang,mhennings/marcohennings-go,d0f/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,scirelli/scirelli-go,bryanxu/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,webfd/go-zh,bryanxu/go-zh,mhennings/marcohennings-go,webfd/go-zh,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,scirelli/scirelli-go,webfd/go-zh,scirelli/scirelli-go,bryanxu/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,scirelli/scirelli-go,scirelli/scirelli-go,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,mhennings/marcohennings-go,scirelli/scirelli-go,sanjosh/sanjos100-tipc