Extensive finalization tests

[kostyantyn]

We have basic test coverage of finalization. We need to create a list of extensive finalization scenarios that we need to cover.

• [ ] Test that finalization is happening at a right checkpoint and the epoch has the right length. Currently, if it's broken only snapshot tests can catch it
• [ ] Test that re-org is possible before justification
• [ ] Test that finalizer commits are accepted when they use UTXO of the block which was pruned
• [ ] Test that node can switch to a correct fork once it discovered that its justified chain was built on double-votes.