go/zodb/btree: tests: Don't forget to close storage

NOTE: db was already being closed in the test's code.

go/zodb/btree/btree_test.go

@@ -105,6 +105,10 @@ func TestBTree(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	defer func() {
+		err := stor.Close(); X(err)
+	}()
+
 	db := zodb.NewDB(stor, &zodb.DBOptions{})
 	defer func() {
 		err := db.Close(); X(err)
@@ -118,8 +122,6 @@ func TestBTree(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	// XXX close db/stor
-
 	// go through small test Buckets/BTrees and verify that Get(key) is as expected.
 	for _, tt := range smallTestv {
 		xobj, err := conn.Get(ctx, tt.oid)