test: commentary for [h-m]*.go

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/5674112
diff --git a/test/map.go b/test/map.go
index c7f1d05..6dec0df 100644
--- a/test/map.go
+++ b/test/map.go
@@ -4,6 +4,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// Test maps, almost exhaustively.
+
 package main
 
 import (