feat(wasm-utxo): refactor bip32 tests for clarity and consistency

Refactors BIP32 tests to improve readability and maintainability by:
- Adding reusable test fixtures for common values
- Creating helper functions for buffer comparison and validation
- Consolidating redundant test assertions
- Simplifying test structure with more consistent patterns
- Improving organization of test cases

Issue: BTC-2650

Co-authored-by: llm-git <llm-git@ttll.de>

Author: OttoAllmendinger