cmd/pack: provide executable name in TestHello
cmd/pack: provide executable name in TestHello

otherwise go build command adds .exe suffix

Fixes issue 7362

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/66250043

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