tests: drivers: mbox: mbox_error_cases: Fix for BUS FAULT on nrf54l15

mbox_error_cases tests must be run with the nordic-flpr snippet
to prevent bus fault error while accessing VPR registers.

Signed-off-by: Jakub Zymelka <jakub.zymelka@nordicsemi.no>

Signed-off-by: Sebastian Głąb <sebastian.glab@nordicsemi.no>