Add test_proposer_in_committee_without_participation__zero_pre_balance

[hwwhww]

Suggested by @dapplion, this is a spec test for proposer in sync committee with balance close to zero, and doesn't participate.

Testing target

In process_sync_aggregate

consensus-specs/specs/altair/beacon-chain.md

Lines 549 to 559 in 2faa44b

	proposer_reward = Gwei(participant_reward * PROPOSER_WEIGHT // (WEIGHT_DENOMINATOR - PROPOSER_WEIGHT))
	# Apply participant and proposer rewards
	all_pubkeys = [v.pubkey for v in state.validators]
	committee_indices = [ValidatorIndex(all_pubkeys.index(pubkey)) for pubkey in state.current_sync_committee.pubkeys]
	for participant_index, participation_bit in zip(committee_indices, sync_aggregate.sync_committee_bits):
	if participation_bit:
	increase_balance(state, participant_index, participant_reward)
	increase_balance(state, get_beacon_proposer_index(state), proposer_reward)
	else:
	decrease_balance(state, participant_index, participant_reward)

We have implemented a for-loop designed to go through the committee sequentially. It either increases or decreases the balances in the for-loop. The sequence in which these adjustments are applied is critical, as it significantly influences the final results.

Test case description

In this test case, we have (1) proposer in sync committee, (2) only "the validator next to the proposer in committee" has participated.

The proposer balance should get decreased and hit the lower bound 0 first, and then, increased with proposer reward in the next iteration on the participant.

As expected, the result doesn't match our testing helper check (validate_sync_committee_rewards.

Note that since we use effective balance to seed the proposer, the best I can mock in the test is setting state.balances[proposer_index] but not setting state.validators[proposer_index].effective_balance.

[dapplion]

I'm not sure if this approach triggers the condition. If I understand you code, there is exactly one committee participant, which is the proposer. The proposer starts with 0 balance, and gets rewarded for participating in the sync committee, but there's no balance decrease for the propose.

To trigger the condition we need to (1) have the non-participation penalty of the proposer index saturate to 0, and have a later participant reward increase the proposer balance to > 0. So we should set the proposer participation to False and at least one bit to True whose bit index is greater than the proposer location in the sync committee.