go/zodb/demo: Test it on all py2/py3 ZODB kinds of data we care about

Similarly to FileStorage, fs1tools and zodbtools previously we were testing
demo only with old FileStorage testdata generated by python and pickle
protocol=2. However even on py2 there are more pickle protocols that
are in use, and also there is python3.

-> Adjust demo testing to automatically load and test agains all ZODB
kinds from recently updated FileStorage testdata.

All py2_pickle1, py2_pickle2, py2_pickle3 and py3_pickle3 are handled well out of the box.

go/zodb/storage/demo/demo_test.go

-// Copyright (C) 2021  Nexedi SA and Contributors.
-//                     Kirill Smelkov <kirr@nexedi.com>
+// Copyright (C) 2021-2024  Nexedi SA and Contributors.
+//                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
 // it under the terms of the GNU General Public License version 3, or (at your
@@ -160,10 +160,21 @@ func TestEmptyDB(t *testing.T) {
 	})
 }
 
+// ztestdataReg keeps registry of ZODB test data we use in tests with non-empty Preload.
+// we take the data from fs1 testdata.
+var  ztestdataReg = xtesting.ZTestDataRegistry[struct{}]{}
+type ZTestData    = xtesting.ZTestData[struct{}]
+func init() {
+        ztestdataReg = xtesting.LoadZTestData("../fs1/testdata")
+}
+
 func TestLoad(t *testing.T) {
+	ztestdataReg.RunWithEach(t, _TestLoad)
+}
+func _TestLoad(t *testing.T, z *ZTestData) {
 	X := xtesting.FatalIf(t)
 
-	data := "../fs1/testdata/1.fs"
+	data := z.Path("1.fs")
 	txnvOk, err := xtesting.LoadDBHistory(data); X(err)
 
 	withDemo(t, func(t *testing.T, _ *DemoData, ddrv *Storage) {