Untested Merge spec code from coverage report

[hwwhww]

With --fork=merge flag

• if is_execution_enabled(state, block.body):: the condition was never false
  • Add a test to cover the case of is_execution_enabled result is false #2723
• Validator guide helpers
  • get_pow_block_at_terminal_total_difficulty: Fix get_pow_block_at_terminal_total_difficulty and add unit tests #2726
  • get_terminal_pow_block
  • prepare_execution_payload
  • get_execution_payload