selftests: mlxsw: Add blackhole_nexthop trap test

Test that packets hitting a blackhole nexthop are trapped to the CPU
when the trap is enabled. Test that packets are not reported when the
trap is disabled.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>