Enable process_randao

[hwwhww]

What is wrong?

process_randao is implemented in #252, but it has not been enabled in SerenityStateTransition.

How can it be fixed

• Uncomment

  trinity/eth2/beacon/state_machines/forks/serenity/state_transitions.py

  Line 101 in b6667b6

  # TODO: state = process_randao(state, block, self.config)

• Add helpers in eth2/beacon/tools/builder/validator.py to set randao reveals for test_demo.py.