Verify `back_path_is_valid` for skip lists
Verify the back_path_is_valid
property in https://nomadic-labs.gitlab.io/coq-tezos-of-ocaml/docs/proofs/skip_list_repr
Verify the back_path_is_valid
property in https://nomadic-labs.gitlab.io/coq-tezos-of-ocaml/docs/proofs/skip_list_repr