Randomize validator index in partial withdrawal test

[ensi321]

Background

Raised by @pk910 in discord that Lodestar forked off in devnet 2 on partial withdrawals. After debugging, it was because Lodestar incorrectly checks validator with index state.next_withdrawal_index instead of withdrawal.index for partial withdrawal eligibility in get_expected_withdrawals.

See discord discussion for detail.

This bug is not caught by partial withdrawal spec test because every test case uses fixed validator index
validator_index = spec.get_active_validator_indices(state, current_epoch)[0]
which is 0 and it coincides with the default value of state.next_withdrawal_index which is 0 as well.

Proposal

Make each test pick validator index at random ie.

validator_index = rng(spec.get_active_validator_indices(state, current_epoch))

[mkalinin]

Do we leave any test with 0 index? Might be helpful for testing as it is one of the boundary values

[ensi321]

Do we leave any test with 0 index? Might be helpful for testing as it is one of the boundary values

That's a good point. Updated.

[ralexstokes]

makes sense!

[hwwhww]

lgtm. Thank you @ensi321!