tests: encoding/decoding of covenants

[kilpatty]

We have all of the encoding and decoding implemented for covenants, but due to the differences between rsd and hsd, it's super important that we have tests to ensure that we are properly encoding and decoding these types.

It's also very important for us to implement some edge cases in these scenarios. We should be testing scenarios in which the size of what we are encoding goes above the usual "varint" max before it adds a prefix (252 I believe). That way we can ensure that we are properly using that type.