cmd/go: add testdata/timeoutbench_test.go

This file was accidentally omitted from the update to the final Go 1.8.

From-SVN: r245814

libgo/go/cmd/go/testdata/timeoutbench_test.go

+package timeoutbench_test
+
+import (
+	"testing"
+	"time"
+)
+
+func BenchmarkSleep1s(b *testing.B) {
+	time.Sleep(1 * time.Second)
+}