tests/bsim: Provisionally disable the nrf54l15 BT tests

Let's provisionally disable the nrf54l15 BT tests in CI
due to instability issues in the controller for this
platform.
See https://github.com/zephyrproject-rtos/zephyr/issues/74635



This should be reverted once the underlaying issue is fixed.

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