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

pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,tv42/old-go,dsriram/golang,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,tv42/old-go,tv42/old-go,apparentlymart/golang,dsriram/golang,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,tv42/old-go,gnanderson/go,gnanderson/go,apparentlymart/golang