Improve Altair pytest coverage

[hwwhww]

• [ ] [phase0] beacon
  • slash_validator: whistleblower_index was never None
    • this field is used by custody game process_challenge_deadlines, but currently, the custody tests are not enabled.
• [ ] [phase0] fork choice tests (#2487)
  • some conditions were never be satisfied or be false.
  • [x] filter_block_tree never returns False
  • [x] should_update_justified_checkpoint never returns True
  • [x] update_latest_messages: condition i not in store.latest_messages or target.epoch > store.latest_messages[i].epoch was never False
  • [ ] on_block: state.current_justified_checkpoint.epoch > store.justified_checkpoint.epoch was never False
    • edited: currently commented out
• [ ] [phase0] validator
  • get_committee_assignment never returns None
  • get_aggregate_signature is uncovered
• [ ] [phase0] weak subjectivity
  • the whole spec is untested
• [x] [altair] validator & networking (#2499, #2505)
  • some helper functions are uncovered
  • get_sync_committee_message
  • get_sync_committee_selection_proof
  • is_sync_committee_aggregator
  • get_contribution_and_proof
  • get_contribution_and_proof_signature
  • get_sync_subcommittee_pubkeys

[hwwhww]

I'd pick fork choice & validator guide tests. 👋

[djrtwo]

I did a pass on coverage needed in state transition and am documenting progress here:

https://docs.google.com/spreadsheets/d/1CbJy3iCRfXnG_jclMrDpktE-gEOXP9nAtGzoUYmuLVM/edit#gid=0

[leolara]

I am closing this issue because it seems stale. Please, do not hesitate to reopen it if this is a mistake