tests: Add test scenario for MCP23009E I/O expander.

Add a YAML test scenario file `tests/scenarios/mcp23009e.yaml` for the MCP23009E driver.

### Expected tests
- **Mock**: verify default register values (IODIR, GPIO)
- **Hardware**: configure pins, read/write GPIO state
- **Manual**: validate pin output with visual check (LED)

### Reference
- Driver: `lib/mcp23009e/`
- Test framework: PR #28