os: add Expand benchmarks

Change-Id: I68e65591cc50433f97a97027e3ae3b452451adf2
Reviewed-on: https://go-review.googlesource.com/106696
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/env_test.go

@@ -62,6 +62,22 @@ func TestExpand(t *testing.T) {
 	}
 }
 
+func BenchmarkExpand(b *testing.B) {
+	var s string
+	b.Run("noop", func(b *testing.B) {
+		b.ReportAllocs()
+		for i := 0; i < b.N; i++ {
+			s = Expand("tick tick tick tick", func(string) string { return "" })
+		}
+	})
+	b.Run("multiple", func(b *testing.B) {
+		b.ReportAllocs()
+		for i := 0; i < b.N; i++ {
+			s = Expand("$a $a $a $a", func(string) string { return "boom" })
+		}
+	})
+}
+
 func TestConsistentEnviron(t *testing.T) {
 	e0 := Environ()
 	for i := 0; i < 10; i++ {