tests/subsys/logging/log_backend_fs: Simplify and enable for native_sim

* Simplify the test definition by using an EXTRA DTC overlay
  so we don't need one test per board
* Move native_posix overlays to native_sim
* Switch the default integration platform to native_sim
  from native_posix

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