tests: Add test scenario for APDS-9960 gesture and color sensor.

Add a YAML test scenario file `tests/scenarios/apds9960.yaml` for the APDS-9960 driver.

### Expected tests
- **Mock**: verify device ID register
- **Hardware**: read ambient light, proximity
- **Manual**: validate displayed light/proximity values

### Reference
- Driver: `lib/apds9960/`
- Test framework: PR #28