tests: drivers: dac channel structure has a buffered parameter boolean

Add the '.buffered' field to the dac_channel_cfg structure
when testing the DAC.
This boolean parameter is initialised to 'true' to PASS the test.
It follows the https://github.com/zephyrproject-rtos/zephyr/pull/57730


Also changed for the samples/drivers/dac

Signed-off-by: Francois Ramu <francois.ramu@st.com>