private/testplanet: shutdown databases in reverse order

Since we have caches on top of databases and they are included in the
databases list, we need to shut them down in-reverse order to avoid
issues with flushing to a closed database.

Change-Id: I3f23a527a2a5425638b1a7e2cab84741f019d493

private/testplanet/planet.go

@@ -274,7 +274,8 @@ func (planet *Planet) Shutdown() error {
 		errlist.Add(peer.Close())
 	}
 
-	for _, db := range planet.databases {
+	for i := len(planet.databases) - 1; i >= 0; i-- {
+		db := planet.databases[i]
 		errlist.Add(db.Close())
 	}