unit_testing: Fix NATIVE_APPLICATION kconfig dependency

NATIVE_APPLICATION is deprecated, let's not use it, let's use instead
NATIVE_BUILD.
unit_testing is not really meant to be covered by NATIVE_APPLICATION,
or NATIVE_BUILD for that matter, as these options are meant for POSIX
arch based targets.
Still, some kconfig options will default to something which will
also be incompatible with unit_testing (primarily CONFIG_PICOLIBC=y)
if we don't set at least NATIVE_BUILD, which would cause some unit
tests to fail. So let's select NATIVE_BUILD to avoid needing to add
extra dependencies on the libCs options.

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