tests|scripts/net: Change native_posix references to native_sim

* As we are replacing native_posix with native_sim, let's
  refer to native_sim instead of native_posix in the comments
  of why we have 1 extra interface.

* scripts/net/run-sample-tests.sh builds for native_sim now,
  not native_posix => let's fix it

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