Negative tests for Block::is_valid

[aszepieniec]

• 1. Previous_block` is consistent with current block
  • a) Block height is previous plus one
  • b) Block header points to previous block
  • c) block mmr updated correctly
  • d) Block timestamp is greater than previous block timestamp
  • e) Target difficulty and cumulative proof-of-work were updated correctly
  • f) Block timestamp is less than host-time (utc) + 2 hours.
• 2. Block is valid
  • a) block proof is valid
  • b) max block size is not exceeded
• 3. The transaction is valid
  • a) Verify that MS removal records are valid, done against previous mutator_set_accumulator,
  • b) Verify that all removal records have unique index sets
  • c) verify that we can add mutator_set_update to previous mutator_set_accumulator,
  • c') and that it results in new block's mutator_set_accumulator
  • e) transaction timestamp <= block timestamp
  • f) transaction coinbase <= miner reward
  • g) transaction is valid (internally consistent)

[Sword-Smith]

This function should return a result type with a custom error type, not just a boolean. That will make these negative tests much more robust.