sample: drivers: counter: alarm: adapt test to driver change

Due to counter driver implementation change
"driver/counter/counter_ll_stm32_rtc.c: Add 1 tick to alarm"
It is necessary to adapt sample test (sanitycheck)
to take into consideration 1 tick precision/tolerance.

Signed-off-by: Alexandre Bourdiol <alexandre.bourdiol@st.com>
2 files changed