Make.cmd: also clean _test*
Make.cmd: also clean _test*

Fixes issue 1593.

R=golang-dev, rsc1
CC=golang-dev
http://codereview.appspot.com/4266050

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