samples: drivers: counter: alarm: add stm32-rtc test case

A specific test case for STM32 RTC is needed because of the way Counter
node is selected in preprocessing.
Tests STM32 RTC_Alarm using counter_ll_stm32_rtc driver.
This also clarifies how STM32 RTC Alarm can be used in this test.

Signed-off-by: Abderrahmane Jarmouni <abderrahmane.jarmouni-ext@st.com>