selftests: forwarding: lib.sh: Add start_tcp_traffic

Extract a helper __start_traffic() configurable by protocol type. Allow
passing through extra mausezahn arguments. Add a wrapper,
start_tcp_traffic().

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