[nrf fromtree] samples: boards: nordic: coresight_stm: Fix nrfutil tr…

…ace 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 <[email protected]>
(cherry picked from commit 153f5b6)