test/chan/doubleselect: fix various races
test/chan/doubleselect: fix various races

There were duplicate closes and missing closes,
with the result that the program was rarely testing
as much as it seemed to be.  Now it finishes.

R=r
CC=golang-dev
http://codereview.appspot.com/4008046

wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang