Add tests for parsing of human readable prefixes from text.

[jonathanknowles]

This PR adds tests relating to the parsing of Bech32 human-readable prefixes (HRPs) from text.

In particular, we test that:

• [x] HRPs of incorrect lengths are handled correctly.
• [x] HRPs containing invalid characters are handled correctly.
• [x] known-good HRPs parse successfully.
• [x] known-bad HRPs fail to parse.