light-client: Verification Testing

[brapse]

part of #320

The verifier is composed of multiple predicates, each of which should be tested for both the positive and negative case using concrete types. operations should be mocked out.

• validator_sets_match
• next_validators_match
• header_matches_commit
• valid_commit
• is_within_trust_period
• is_monotonic_bft_time
• is_monotonic_height
• has_sufficient_voting_power (related light-client: consider optimizing voting power calculation by stopping after verifying +1/3 or +2/3  #235)
• has_sufficient_validators_overlap
• has_sufficient_signers_overlap
• valid_next_validator_set

Setup a simple test harness which facilitates to construction of concrete types to limit the dependency graph and make room for more general constructions.

[brapse]

So this as it turns out is trickier than it seems. In order to effectively exercise the predicates mentioned above we need to actually construct concrete types which are very complex. One approach would be to provide factories for these concrete types which extend the fixtures provides in the json tests but allow overriding specific fields. This way the tests can focus on specific field interactions without getting lost in details.