soc: stm32: Enable Debug in stop mode when ZTEST is used

Force STM32_ENABLE_DEBUG_SLEEP_STOP option when running tests.
This option enables Debug in Sleep/stop states and disables it when
disabled (default state).

When disabled, it may be impossible to flash the device with runner
such as openocd. It's generally working using cubeprogrammer, but it might
fail as well with fault configuration.

Instead of fixing each test or breaking CI each time a new test is created
with CONFIG_PM=y, force this option to be enabled when ZTEST=y (as it was
already the case when DEBUG=y).

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>