samples/boards: stm32 pm blinky: Run with twister device testing

Add minimum harness and to get the sample passed when run using
twister.
Adding console validates uart init with DEVICE_RUTIME=y.

Additionally, clarify comment about DEBUG activation.

Fixes #35033

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>
3 files changed