tests: drivers: adc: adc_api: boards: turn of time slicing

set CONFIG_TIMESLICE_SIZE to 0 to disable time slicing.

Certain boards like nucleo_g071rb, nucleo_f091rc,
nucleo_l073rz will not start consecutive samplings as fast
as possible.

Signed-off-by: Fabrice DJIATSA <fabrice.djiatsa-ext@st.com>