testing: add test for issue 5599

Update #5599

R=golang-dev, r, minux.ma
CC=golang-dev
https://golang.org/cl/9738052

src/pkg/testing/benchmark_test.go

+// Copyright 2013 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 testing_test
+
+import (
+	"testing"
+)
+
+var roundDownTests = []struct {
+	v, expected int
+}{
+	{1, 1},
+	{9, 1},
+	{10, 1},
+	{11, 10},
+	{100, 10},
+	//	{101, 100}, // issue 5599
+	{1000, 100},
+	//	{1001, 1000}, // issue 5599
+}
+
+func TestRoundDown10(t *testing.T) {
+	for _, tt := range roundDownTests {
+		actual := testing.RoundDown10(tt.v)
+		if tt.expected != actual {
+			t.Errorf("roundDown10: expected %v, actual %v", tt.expected, actual)
+		}
+	}
+}

src/pkg/testing/export_test.go

+// Copyright 2013 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 testing
+
+var RoundDown10 = roundDown10