Add test coverage that ensures edge cases lead to invalid blocks

[daira]

Let's add test coverage that ensures these edge cases lead to invalid blocks:

• Blocks:
  • 0 txns
  • non-coinbase initial txn
  • multiple coinbase txns
  • unbalanced fees (note: Doesn't Zcash NU5 mainnet allow unbalanced fees where coinbase claims less than the total allowed? Probably safe to ignore unless there were any possible security impacts for that edge case.)
  • contains "context-free-invalid" txns
  • contains txns with nonexistent (e.g. out-of-BCContext inputs)
  • contains txns with multi-spends (e.g. >1 txn with the same input).

Originally posted by @nathan-at-least in #12 (comment)

[daira]

Doesn't Zcash NU5 mainnet allow unbalanced fees where coinbase claims less than the total allowed? Probably safe to ignore unless there were any possible security impacts for that edge case.

Ignoring this was an intentional simplification. Zcash currently doesn't have a fee field in transactions. But it probably will have a fee field as of NU6. At that point we can strengthen the balance rule to prohibit claiming less than allowed.