net: fix TestDialTimeout on windows builder

I don't know what's out there, but something
is answering to 127.0.71.111:80 on our builder,
so use a different port.

Also insert a check that the dial fails, which
would have diagnosed this problem.

Fixes #3016.

R=golang-dev, mikioh.mikioh, r
CC=golang-dev
https://golang.org/cl/5754062
1 file changed