test: enforce 1 proc in the test

otherwise it fails spuriously with "newfunc allocated unexpectedly" message
when run with GOMAXPROCS>1 (other goroutine allocates).

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6347056

test/closure.go

@@ -81,6 +81,7 @@ func h() {
 func newfunc() func(int) int { return func(x int) int { return x } }
 
 func main() {
+	runtime.GOMAXPROCS(1)
 	var fail bool
 
 	go f()