Improve general test coverage

[eddyashton]

We use 3rd party test cases through harness.cpp, but many are disabled. Some for good reasons - gas tracking, oversized exponents - but others fail for unknown reasons.

We should investigate these remaining failures, and also see if there's anything to gain from pulling in more recent test cases. If we find significant untested holes, we should add our own test scenarios to cover these validated against another implementation.