test: change bug040 to use errorcheck

Because bug040.go was ignoring all error messages, the fact
that it got an error about fuction main was being ignored.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5675085

test/fixedbugs/bug040.go

-// ! $G $D/$F.go >/dev/null
-// # ignoring error messages...
+// errorcheck
 
 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
@@ -7,5 +6,6 @@
 
 package main
 
-func main (x, x int) {  // BUG redeclaration error
+func f (x,		// GCCGO_ERROR "previous"
+	x int) {	// ERROR "redeclared|redefinition" "duplicate"
 }