benchmark repro for empty memprofile output

bench_test.go

// On an ubuntu VM run as:
// GOGC=off ./benchtest.test  -test.memprofile=mem.prof -test.run=none -test.bench=. -test.memprofilerate=1
// This will produce a 4k mem.prof with no information in it:
//     $ go tool pproof ./benchtest.test mem.prof
//     (prof) top10
//     Total: 0.0 MB

package benchtest

import (
	"testing"
)

type T struct {
	a int
	b int
}

var a []T

// allocate something
func add() {
	a = append(a, T{1, 2})
}

func BenchmarkTest(b *testing.B) {
	for i := 0; i < b.N; i ++ {
		add()
	}
}