tests/net: For nrf52_bsim use flash model

Now that the nrf52 HW models support it,
let's use their flash model instead of falling back
to the openthread RAM settings backend.

Note that the flash model defaults to just keeping the
flash content in the Linux process heap.

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