os/exec: add BenchmarkExecEcho

Change-Id: Ie955cdc505766447f70b8f262160fe05b60a5b0c
Reviewed-on: https://go-review.googlesource.com/c/164959
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/exec/bench_test.go

+// Copyright 2019 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 exec
+
+import (
+	"testing"
+)
+
+func BenchmarkExecEcho(b *testing.B) {
+	b.ReportAllocs()
+	path, err := LookPath("echo")
+	if err != nil {
+		b.Fatalf("could not find echo: %v", err)
+	}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		if err := Command(path).Run(); err != nil {
+			b.Fatalf("echo: %v", err)
+		}
+	}
+}