Asserts: Add ConsensusOutput validations

The validation of the `ConsensusTests` also includes validations for the `ConsensusOutput` beyond the `ConsensusRound`. For example, we can check that all ancient events either reached consensus or were reported as stale.

Start by locating ConsensusTests and its existing ConsensusRound validation, then inspect how ConsensusOutput is represented and used in those tests. Add validations for the stated ConsensusOutput conditions, including ensuring ancient events either reached consensus or were reported as stale, and run the relevant ConsensusTests to verify the assertions.

Written by the indexing model from the issue text.