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

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