selftests/tc-testing: Add a test case for FQ_CODEL with DRR parent

Add a test case for FQ_CODEL with DRR parent to verify packet drop
behavior when the queue becomes empty. This helps ensure proper
notification mechanisms between qdiscs.

Note this is best-effort, it is hard to play with those parameters
perfectly to always trigger ->qlen_notify().

Cc: Pedro Tammela <pctammela@mojatatu.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Link: https://patch.msgid.link/20250403211636.166257-5-xiyou.wangcong@gmail.com
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: congwang

tools/testing/selftests/tc-testing/tc-tests/infra/qdiscs.json

@@ -251,5 +251,36 @@
             "$TC qdisc del dev $DUMMY handle 1: root",
             "$IP addr del 10.10.10.10/24 dev $DUMMY || true"
         ]
+    },
+    {
+        "id": "a4c0",
+        "name": "Test FQ_CODEL with DRR parent - force packet drop with empty queue",
+        "category": [
+            "qdisc",
+            "fq_codel",
+            "drr"
+        ],
+        "plugins": {
+            "requires": "nsPlugin"
+        },
+        "setup": [
+            "$IP link set dev $DUMMY up || true",
+            "$IP addr add 10.10.10.10/24 dev $DUMMY || true",
+            "$TC qdisc add dev $DUMMY handle 1: root drr",
+            "$TC class add dev $DUMMY parent 1: classid 1:10 drr quantum 1500",
+            "$TC qdisc add dev $DUMMY parent 1:10 handle 10: fq_codel memory_limit 1 flows 1 target 0.1ms interval 1ms",
+            "$TC filter add dev $DUMMY parent 1: protocol ip prio 1 u32 match ip protocol 1 0xff flowid 1:10",
+            "ping -c 5 -f -I $DUMMY 10.10.10.1 > /dev/null || true",
+            "sleep 0.1"
+        ],
+        "cmdUnderTest": "$TC -s qdisc show dev $DUMMY",
+        "expExitCode": "0",
+        "verifyCmd": "$TC -s qdisc show dev $DUMMY | grep -A 5 'qdisc fq_codel'",
+        "matchPattern": "dropped [1-9][0-9]*",
+        "matchCount": "1",
+        "teardown": [
+            "$TC qdisc del dev $DUMMY handle 1: root",
+            "$IP addr del 10.10.10.10/24 dev $DUMMY || true"
+        ]
     }
 ]