selftests: forwarding: lib: Add start_traffic_pktsize() helpers

Add two helpers, start_traffic_pktsize() and start_tcp_traffic_pktsize(),
that allow explicit overriding of packet size. Change start_traffic() and
start_tcp_traffic() to dispatch through these helpers with the default
packet size.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>