selftests/tc-testing: Add a test case for pfifo_head_drop qdisc when limit==0

When limit == 0, pfifo_tail_enqueue() must drop new packet and
increase dropped packets count of the qdisc.

All test results:

1..16
ok 1 a519 - Add bfifo qdisc with system default parameters on egress
ok 2 585c - Add pfifo qdisc with system default parameters on egress
ok 3 a86e - Add bfifo qdisc with system default parameters on egress with handle of maximum value
ok 4 9ac8 - Add bfifo qdisc on egress with queue size of 3000 bytes
ok 5 f4e6 - Add pfifo qdisc on egress with queue size of 3000 packets
ok 6 b1b1 - Add bfifo qdisc with system default parameters on egress with invalid handle exceeding maximum value
ok 7 8d5e - Add bfifo qdisc on egress with unsupported argument
ok 8 7787 - Add pfifo qdisc on egress with unsupported argument
ok 9 c4b6 - Replace bfifo qdisc on egress with new queue size
ok 10 3df6 - Replace pfifo qdisc on egress with new queue size
ok 11 7a67 - Add bfifo qdisc on egress with queue size in invalid format
ok 12 1298 - Add duplicate bfifo qdisc on egress
ok 13 45a0 - Delete nonexistent bfifo qdisc
ok 14 972b - Add prio qdisc on egress with invalid format for handles
ok 15 4d39 - Delete bfifo qdisc twice
ok 16 d774 - Check pfifo_head_drop qdisc enqueue behaviour when limit == 0

Signed-off-by: Quang Le <quanglex97@gmail.com>
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Link: https://patch.msgid.link/20250204005841.223511-3-xiyou.wangcong@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: quanglex97

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

@@ -313,6 +313,29 @@
         "matchPattern": "qdisc bfifo 1: root",
         "matchCount": "0",
         "teardown": [
+	]
+    },
+    {
+        "id": "d774",
+        "name": "Check pfifo_head_drop qdisc enqueue behaviour when limit == 0",
+        "category": [
+            "qdisc",
+            "pfifo_head_drop"
+        ],
+        "plugins": {
+            "requires": "nsPlugin"
+        },
+        "setup": [
+            "$IP addr add 10.10.10.10/24 dev $DUMMY || true",
+            "$TC qdisc add dev $DUMMY root handle 1: pfifo_head_drop limit 0",
+            "$IP link set dev $DUMMY up || true"
+        ],
+        "cmdUnderTest": "ping -c2 -W0.01 -I $DUMMY 10.10.10.1",
+        "expExitCode": "1",
+        "verifyCmd": "$TC -s qdisc show dev $DUMMY",
+        "matchPattern": "dropped 2",
+        "matchCount": "1",
+        "teardown": [
         ]
     }
 ]