bug264 didn't report BUG correctly, caused "fail" from test/run

R=rsc, gri
CC=golang-dev
https://golang.org/cl/762041

test/bugs/bug264.go

-// $G $D/$F.go && $L $F.$A && ./$A.out
+// $G $D/$F.go && $L $F.$A && ./$A.out || echo BUG
 
 // Copyright 2010 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
@@ -38,12 +38,12 @@ var _, _ = bal() // bal is called twice
 
 func main() {
 	if fooCount != 1 {
-		panic("BUG: fooCount != 1")
+		panic("fooCount != 1")
 	}
 	if barCount != 1 {
-		panic("BUG: barCount != 1")
+		panic("barCount != 1")
 	}
 	if balCount != 1 {
-		panic("BUG: balCount != 1")
+		panic("balCount != 1")
 	}
 }

test/golden.out

@@ -189,5 +189,6 @@ bar
 bar
 bal
 bal
-BUG: barCount != 1
+barCount != 1
 panic PC=xxx
+BUG