go/internal/xtesting: Switch to updated FileStorage testdata to validate LoadDBHistory

Old fs1/testdata/1.fs will be going away -> we need to switch to using
newly generated multi-zkind testdata. However for this test using only
one zkind should be enough. Let's pick what we currently primarily use:
py2_pickle3.

go/internal/xtesting/xtesting_test.go

-// Copyright (C) 2020  Nexedi SA and Contributors.
+// Copyright (C) 2020-2024  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -29,7 +29,7 @@ import (
 
 func TestLoadDBHistory(t *testing.T) {
 	X := FatalIf(t)
-	data := "../../zodb/storage/fs1/testdata/1.fs"
+	data := "../../zodb/storage/fs1/testdata/py2_pickle3/1.fs" // one zkind is enough for hereby test
 
 	txnv, err := LoadDBHistory(data); X(err)