Tracking issue: more test cases

We iterated over the light client on happy path and testing over valid data. We now need to add more test cases to our code to ensure it's proper behavior in all cases.

_To evolve along development_

- [ ] Emulate state transition from genesis to epoch 4 with data being valid
- [ ] Receiving a dishonest proof of epoch transition (eg: wrong signature, past epoch ...)
- [ ] Receiving a dishonest sparse merkle proof
- [ ] `EpochChangeProof` and `LedgerInfoWithSignatures` that skip an epoch

Start by locating the existing Rust light-client tests for the happy path and valid data, then run them to understand the current coverage. Add focused cases for genesis-to-epoch-4 transitions, dishonest epoch-transition and sparse-Merkle proofs, and skipped epochs in EpochChangeProof and LedgerInfoWithSignatures; done means these scenarios are covered and pass.

Written by the indexing model from the issue text.