Add sanity unit test for validator guide + update validator guide

[hwwhww]

1. Add unit tests for validator guide to improve test coverage. (Fix Add tests for executable validator guide #1352)
   • phase 0 spec.py test coverage raises from 93% to 99%
   • note that these tests are not part of client test vectors
2. Validator guide update:
   1. Avoid negative computation in is_candidate_block
   2. Fix get_block_signature: avoid extra casting; it's simpler to use BeaconBlock instead of BeaconHeader.
   3. Make compute_new_state_root a pure function.
3. Update minimal.config: Increase EPOCHS_PER_ETH1_VOTING_PERIOD from 2 to 4 for testing eth1 votes consensus vote

[djrtwo]

nice! finally, we won't introduce silly errors when updating v-guide and this should help in the phase 1 v-guide dev too :)

[hwwhww]

@djrtwo thanks for the review!

Changelog

1. Apply PR feedback
2. Update minimal.config: Increase EPOCHS_PER_ETH1_VOTING_PERIOD from 2 to 4 since we need more than 2 epochs for testing test_get_eth1_vote_consensus_vote.
3. Make compute_new_state_root a pure function (Add sanity unit test for validator guide + update validator guide #1745 (comment))