test: float to integer test case

gccgo currently fails this test:

fixedbugs/bug402.go:12:9: error: floating point constant truncated to integer
fixedbugs/bug402.go:13:8: error: floating point constant truncated to integer

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/5600050

test/fixedbugs/bug402.go

+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "fmt"
+
+var a = []int64{
+	0.0005 * 1e9,
+	0.001 * 1e9,
+	0.005 * 1e9,
+	0.01 * 1e9,
+	0.05 * 1e9,
+	0.1 * 1e9,
+	0.5 * 1e9,
+	1 * 1e9,
+	5 * 1e9,
+}
+
+func main() {
+	s := ""
+	for _, v := range a {
+		s += fmt.Sprint(v) + " "
+	}
+	if s != "500000 1000000 5000000 10000000 50000000 100000000 500000000 1000000000 5000000000 " {
+		panic(s)
+	}
+}