tests(app): ⛓ use mock consensus for IBC tests

[cratelyn]

https://github.com/penumbra-zone/hermes/blob/64ef92934fa3ea399321a61ef1386b0da2aed9db/crates/relayer/src/chain/handle.rs#L376

• Tracking issue: 🤝 mock consensus engine for App tests #3588
• tests: ❓ implement an IBC handshake using mock consensus engine #3758

@zbuc and i spent some time pairing on how he might use the penumbra-mock-consensus library to test the ibc relayer. in order to work with two mock penumbra chains, we may want to implement the ChainHandle trait for TestNode<C>.