Fixed a pack test that was not compatible with storages that always

return an object count of 0.

src/ZODB/tests/PackableStorage.py

@@ -312,7 +312,8 @@ class PackableStorage(PackableStorageBase):
         root[2] = conn.get_connection('o').root()
         transaction.commit()
         db.pack(time.time()+1)
-        self.assertEqual(len(self._storage), 1)
+        # some valid storages always return 0 for len()
+        self.assertTrue(len(self._storage) in (0, 1))
 
     def checkPackAllRevisions(self):
         self._initroot()