test: Reduce race conditions in chan/nonblock.go.
test: Reduce race conditions in chan/nonblock.go.

nonblock.go wants to test nonblocking operations on
synchronous channels, so it is inherently racy.  This
introduces loops to make the race conditions much more likely
to succeed when using gccgo.

R=r
CC=golang-dev
http://codereview.appspot.com/2161043

apparentlymart/golang,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,ssrl/go,dsriram/golang,apparentlymart/golang,gnanderson/go,ssrl/go,gnanderson/go,pombredanne/go-deleteme,h8liu/golang,dsriram/golang,ssrl/go,ssrl/go,tv42/old-go,dsriram/golang,tv42/old-go,h8liu/golang,gnanderson/go,ssrl/go,tv42/old-go,dsriram/golang,h8liu/golang,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,h8liu/golang,dsriram/golang,ssrl/go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,h8liu/golang,tv42/old-go,gnanderson/go,apparentlymart/golang,tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,ssrl/go,h8liu/golang,pombredanne/go-deleteme,h8liu/golang