selftests: mlxsw: Do not hard code trap group name

It can be derived dynamically from the trap's name, so drop it.

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