tests: drivers: dac: add xg23 and xg24 boards to dac tests

Add board overlays and defines to the test sources files to execute
the dac_api and dac_loopback tests on xg23_rb4210a, xg24_dk2601b,
xg24_ek2703a and xg24_rb4187c.

Signed-off-by: Bastien Beauchamp <bastien.beauchamp@silabs.com>