tests/subsys log_backend_fs: Add missing overlays for native_sim

During the hwmv1->v2 transition, overlays from a base
board were made to be shared with the variants.
So at that time all overlays for variants which were
just copies were removed.
After that in
https://github.com/zephyrproject-rtos/zephyr/pull/71149


this shared/merged overlay behaviour was reverted,
but not all tests were fixed.

This is one of those. Let's fix it.

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