write unit tests

## Abstract

Write more unit tests for UR serialization and deserialization.

First, there are small changes that need to be done to the UR test vectors (one has an incorrect depth and another is missing a parent fingerprint), see https://github.com/BlockchainCommons/Research/issues/83. To correct these changes open a PR in Research repo. After approved, write unit tests for those test vectors in this repo.

Tag @wolfmcnally in https://github.com/BlockchainCommons/Research/issues/83 to let him know you'll be working on the issue

You'll need the following tool:
* https://github.com/BlockchainCommons/bytewords-cli

And some general knowledge about:
* URs: https://github.com/BlockchainCommons/Research/blob/master/papers/bcr-2020-005-ur.md
* bytewords: https://github.com/BlockchainCommons/Research/blob/master/papers/bcr-2020-012-bytewords.md
* CBOR (http://cbor.me/)
* PSBT