selftests: mlxsw: Add a test case for devlink-trap

Test generic devlink-trap functionality over mlxsw. These tests are not
specific to a single trap, but do not check the devlink-trap common
infrastructure either.

Currently, the only test case is device deletion (by reloading the
driver) while packets are being trapped.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>