test(chain): test forking before/after finality

[Stebalien]

Related Issues

tests for #12650

Proposed Changes

Test checkpoints beyond finality. Specifically, ensure:

1. We can checkpoint before finality.
2. We cannot fork more than finality epochs.

Additional Info

I had to add a bit of a hack to the chain-gen logic to optionally prevent time (state) from moving forwards. Otherwise, we'd lose all power and actually providing window posts was going to be too much trouble.

Checklist

Before you mark the PR ready for review, please make sure that:

• Commits have a clear commit message.
• PR title conforms with contribution conventions
• Update CHANGELOG.md or signal that this change does not need it per contribution conventions
• New features have usage guidelines and / or documentation updates in
  • Lotus Documentation
  • Discussion Tutorials
• Tests exist for new functionality or change in behavior
• CI is green

[masih]

nice