Fix get_pow_block_at_terminal_total_difficulty and add unit tests

[hwwhww]

Part of #2722

Spec changes

1. get_pow_block_at_terminal_total_difficulty was wrongly iterating pow_chain keys. It should be dict values pow_chain.values() instead of pow_chain.
2. Handle the edge case of block.parent_hash == Hash32() and block.parent_hash not in pow_chain. Without handling it, it would raise KeyErorr in pyspec.
   • New concern: it's the first time to use continue in specs. Is it too Pythonic and difficult to do formal verification? fixed

Testing

Add unit tests for get_pow_block_at_terminal_total_difficulty

[mkalinin]

What if we do it as follows:

if block_reached_ttd:
    if block.parent_hash == Hash32():
        return block
    else:
        parent = pow_chain[block.parent_hash]
        parent_reached_ttd = parent.total_difficulty >= TERMINAL_TOTAL_DIFFICULTY
        if not parent_reached_ttd:
            return block

[mkalinin]

Looks good to me! 👍

[djrtwo]

looks good. minor suggestion