tc-testing: Add control-plane selftest for skbmod SKBMOD_F_ECN option

Recently we added a new option, SKBMOD_F_ECN, to tc-skbmod(8).  Add a
control-plane selftest for it.

Depends on kernel patch "net/sched: act_skbmod: Add SKBMOD_F_ECN option
support", as well as iproute2 patch "tc/skbmod: Introduce SKBMOD_F_ECN
option".

Reviewed-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>
Signed-off-by: David S. Miller <davem@davemloft.net>