tests/drivers/eeprom: Several fixes and improvements

* Fix app path in documentation
* Move the at24 emulator overlays to be general instead of
  native_posix specific
* Switch default test platform from native_posix to native_sim
* From build only test, exclude platforms which are used
  in the build and run test.

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