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>

tools/testing/selftests/tc-testing/tc-tests/actions/skbmod.json

@@ -417,5 +417,29 @@
         "teardown": [
             "$TC actions flush action skbmod"
         ]
+    },
+    {
+        "id": "fe09",
+        "name": "Add skbmod action to mark ECN bits",
+        "category": [
+            "actions",
+            "skbmod"
+        ],
+        "setup": [
+            [
+                "$TC actions flush action skbmod",
+                0,
+                1,
+                255
+            ]
+        ],
+        "cmdUnderTest": "$TC actions add action skbmod ecn",
+        "expExitCode": "0",
+        "verifyCmd": "$TC actions get action skbmod index 1",
+        "matchPattern": "action order [0-9]*: skbmod pipe ecn",
+        "matchCount": "1",
+        "teardown": [
+            "$TC actions flush action skbmod"
+        ]
     }
 ]