Fix broken SnapshotMetaSpec

The `SnapshotMetaSpec` is intermittently failing because duplicate `Identifier`s are sometimes created from the arbitrary
Im not sure i understand the difference between this and using the old implementation. I would of thought `IdentifierList.ids.last` is equivalent to `(1 to n).foldLeft(Identifier.initial)((acc, _) => acc.next.get)`