test: Match gccgo error messages.

bug298.go:10:2: error: expected declaration
bug298.go:10:25: error: expected ‘;’ or newline after top level declaration
bug298.go:10:25: error: expected declaration

R=rsc
CC=golang-dev
https://golang.org/cl/2156046

test/fixedbugs/bug298.go

@@ -7,5 +7,5 @@
 package ddd
 
 func Sum() int
-	for i := range []int{} { return i }  // ERROR "return outside function"
+	for i := range []int{} { return i }  // ERROR "return outside function|expected"