test: match gccgo error messages for bug274.go.

bug274.go:23:3: error: missing statement after label
bug274.go:25:3: error: missing statement after label
bug274.go:28:3: error: label ‘L2’ defined and not used

R=rsc
CC=golang-dev
https://golang.org/cl/4273114

test/fixedbugs/bug274.go

@@ -24,6 +24,7 @@ func main() {
 	case 1:
 		L1:  // ERROR "statement"
 	default:
-		L2:  // correct since no semicolon is required before a '}'
+		     // correct since no semicolon is required before a '}'
+		L2:  // GCCGO_ERROR "not used"
 	}
 }