Convert the DUT class from a traditional class to a dataclass to enable
proper serialization and support for future multi-device testing with
pytest-harness.
Key changes:
- Migrated DUT class to use `dataclass` decorator with proper type hints
- Renamed `baud` property to `serial_baud` for consistency
- Updated hardware map schema to support both `baud` (legacy) and
`serial_baud` fields for backward compatibility
- Updated tests
Signed-off-by: Grzegorz Chwierut <grzegorz.chwierut@nordicsemi.no>
efc36d96d3