tc-testing: add support for sending various scapy packets

It can be worth sending different scapy packets on a given test, as in the
last patch of this series. For that, lets listify the scapy attribute and
simply iterate over it.

Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>