Model tests choke on too early fanout after rollbacks

[ch1bo]

The ModelSpec has RollbackAndForward actions.

This will have the simulated MockChain emit RollBackward and RollForward interactions through the chain layer such that the HydraNode sees time to revert and advance again.

This affects ReadyToFanout indications given through the API.

The ModelSpec uses the sent serverOutputs to waitForReadyToFanout. However, if we see a rollback and forward, the chain is might not be at the recent enough point anymore and submitting a fanoutTx results in LowerBoundBeforeContestationDeadline on-chain verification errors.

Reproduction: Example commit a70a781 with seed 823537679