selftests/tc-testing: Try to add teql as a child qdisc

Add a selftest that attempts to add a teql qdisc as a qfq child.
Since teql _must_ be added as a root qdisc, the kernel should reject
this.

Signed-off-by: Victor Nogueira <victor@mojatatu.com>
Link: https://patch.msgid.link/20260114160243.913069-4-jhs@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: vbnogueira

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

@@ -81,5 +81,30 @@
             "$TC qdisc del dev $DUMMY handle 1: root",
             "$IP link del dev $DUMMY"
         ]
+    },
+    {
+        "id": "124e",
+        "name": "Try to add teql as a child qdisc",
+        "category": [
+            "qdisc",
+            "ets",
+            "tbf"
+        ],
+        "plugins": {
+            "requires": [
+                "nsPlugin"
+            ]
+        },
+        "setup": [
+            "$TC qdisc add dev $DUMMY root handle 1: qfq",
+            "$TC class add dev $DUMMY parent 1: classid 1:1 qfq weight 15 maxpkt 16384"
+        ],
+        "cmdUnderTest": "$TC qdisc add dev $DUMMY parent 1:1 handle 2:1 teql0",
+        "expExitCode": "2",
+        "verifyCmd": "$TC -s -j qdisc ls dev $DUMMY parent 1:1",
+        "matchJSON": [],
+        "teardown": [
+            "$TC qdisc del dev $DUMMY root handle 1:"
+        ]
     }
 ]