adds a test

weed/shell/command_volume_balance.go

@@ -120,7 +120,7 @@ func balanceVolumeServers(commandEnv *CommandEnv, diskTypes []types.DiskType, vo
 
 func balanceVolumeServersByDiskType(commandEnv *CommandEnv, diskType types.DiskType, volumeReplicas map[uint32][]*VolumeReplica, nodes []*Node, volumeSizeLimit uint64, collection string, applyBalancing bool) error {
 
-	// balance readable volumes
+	// balance read only volumes
 	for _, n := range nodes {
 		n.selectVolumes(func(v *master_pb.VolumeInformationMessage) bool {
 			if collection != "ALL_COLLECTIONS" {