Improve testing

[tuturu-tech]

Describe the desired feature

• [x] Fix failing Medusa tests by:
  • [x] Adding a medusa corpus for value transfer
  • [x] Removing non-failing call sequences from the corpora
• [ ] Add unit tests for error handling
• [x] Refactor tests to reduce code duplication
• [x] Add targeted tests for bytes and string decoding
• [x] Add testing guidance to CONTRIBUTING.md
• [x] Refactor tests so test files are generated in a temporary dir and removed upon test completion