Testing framework: baker accounts and register delegate

This MR adds new features related to using a generated account as a baker.

It adds:

1. Test.baker_account : (string * key) -> tez option -> unit: adds an account (sk, pk) as a baker. The change is only effective after Test.reset_state.
2. Test.register_delegate : key_hash -> unit: registers a key_hash corresponding to an account as a delegate.
3. Test.bake_until_n_cycle_end : nat -> unit: it bakes until n cycles pass, so that the pkh` can be used effectively as a baker.

• has a changelog entry