test: match gccgo error messages for init.go

init.go:16:10: error: invalid reference to unexported identifier ‘runtime.init’
init.go:15:2: error: reference to undefined name ‘init’
init.go:17:10: error: reference to undefined name ‘init’

R=golang-dev, rsc1
CC=golang-dev
https://golang.org/cl/4289074

test/init.go

@@ -12,7 +12,7 @@ func init() {
 }
 
 func main() {
-	init()         // ERROR "undefined: init"
+	init()         // ERROR "undefined.*init"
 	runtime.init() // ERROR "unexported.*runtime\.init"
-	var _ = init   // ERROR "undefined: init"
+	var _ = init   // ERROR "undefined.*init"
 }