samples & tests: Replace NATIVE_POSIX_SLOWDOWN_TO_REAL_TIME

Replace NATIVE_POSIX_SLOWDOWN_TO_REAL_TIME with the equivalent for
native_sim NATIVE_SIM_SLOWDOWN_TO_REAL_TIME.

native_sim accepted transitionally NATIVE_POSIX_SLOWDOWN_TO_REAL_TIME
to ease porting from native_posix, but this option is being deprecated.

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