runtime: add a map growth benchmark

Updates #19931
Updates #19992

Change-Id: Ib2d4e6b9b89a49caa443310d896dce8d6db06050
Reviewed-on: https://go-review.googlesource.com/40978
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/mapspeed_test.go

@@ -5,6 +5,7 @@ package runtime_test
 
 import (
 	"fmt"
+	"strconv"
 	"strings"
 	"testing"
 )
@@ -308,6 +309,20 @@ func BenchmarkSmallKeyMap(b *testing.B) {
 	}
 }
 
+func BenchmarkMapPopulate(b *testing.B) {
+	for size := 1; size < 1000000; size *= 10 {
+		b.Run(strconv.Itoa(size), func(b *testing.B) {
+			b.ReportAllocs()
+			for i := 0; i < b.N; i++ {
+				m := make(map[int]bool)
+				for j := 0; j < size; j++ {
+					m[j] = true
+				}
+			}
+		})
+	}
+}
+
 type ComplexAlgKey struct {
 	a, b, c int64
 	_       int