tests: Increase test coverage

[Yawning]

While a lot of the code is well exercised by the test cases, there are a number of gaps that should also be covered if only to prevent regressions when further development occurs. In particular

• The ristretto code could use more test cases, particularly on functions that are just wrappers around the corresponding Ed25519 routines.
• A number of "user-friendlyness" routines in the curve package could use more coverage, such as the constructors and MarshalBinary/UnmarshalBinary routines.

As of right now, none of these routines are used, and they are likely correct, but it won't hurt to make sure they stay correct.