btcutil: add BIP32 Test Vector 5 validation and test cases

[kuliq23]

Change Description

Add stricter BIP32 extended key validation in NewKeyFromString to comply with BIP32 test vector 5, and update tests accordingly.

Just a spec-compliance fix.
Found using bitcoinfuzz tool.
Feel free to refactor or modify the implementation directly if a different approach is preferred.

Steps to Test

1. cd btcutil
2. go test ./hdkeychain

Pull Request Checklist

Testing

• Your PR passes all CI checks. - I believe it is not due to the introduced changes.
• Tests covering the positive and negative (error paths) are included.
• Bug fixes contain tests triggering the bug to prevent regressions.

Code Style and Documentation

• The change is not insubstantial. Typo fixes are not accepted to fight bot spam.
• The change obeys the Code Documentation and Commenting guidelines, and lines wrap at 80.
• Commits follow the Ideal Git Commit Structure.
• Any new logging statements use an appropriate subsystem and logging level. (N/A)

[allocz]

Nice work, ACK 209175a