zip: add a test for the previous >65k files fix
zip: add a test for the previous >65k files fix

This surprisingly takes 30 seconds on my fast machine
so disabling by default. Need to optimize the Writer
at some point.

R=golang-dev, r, r
CC=golang-dev
http://codereview.appspot.com/4815048

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