samples: boards: nordic: coresight_stm: Fix nrfutil trace command

Latest version of nrfutil trace doesn't support setting
uart baudrate.
Remove --baudrate {baudrate_val} from command that executes nrfutil.

Also, combine pytest script for:
- samples.boards.nordic.coresight_stm,
- tests.boards.nrf.coresight_stm,
to minimize code duplication (fix compliance check R0801).

Signed-off-by: Sebastian Głąb <sebastian.glab@nordicsemi.no>