Standalone tests for Shelley forecasting

The goal of this ticket is to add explicit tests ensuring that Shelley/Cardano forecasting behaves as expected by Consensus. This includes:

- The forecasting range works as expected (forecasting within it succeeds, and fails otherwise).
- Cross-epoch forecasting applies updates to e.g. the stake distribution (Ledger recently had a bug where that was not the case, which we could have caught early and avoided a debugging session: https://github.com/input-output-hk/cardano-ledger/pull/3571

Depending on how realistic we want this to be, we might want to generate e.g. delegation transactions; we should investigate what the status quo in Ledger for things like this is.