samples/board/stm32: PM blinky: Enable CONFIG_DEBUG in CI

Enable CONFIG_DEBUG when test is run in CI.
This will allow DGB access in stop modes (and specially flashing)
and avoid potential issues when test in run in a test suite.

Note that on some targets (stm32wb for instance) openocd is able
to do the same operation, and which allows to reflash the target
even if switch was not enabled, but this could not be the case
on all targets.

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