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

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