tests/drivers/adc: Switch to native_sim

Switch from native_posix to native_sim as default test platform.
For test with overlays, switch overlays and platform_allow from
native_posix to native_sim.
For tests without overlays, enable also in native_sim, and
set native_sim as integration platform.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>