tc-testing: fix list handling

python lists don't have an 'add' method, but 'append'.

Fixes: 14e5175e ("tc-testing: introduce scapyPlugin for basic traffic")
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>