selftests: tc-testing: add one test for flushing explicitly created chain

Add the test for additional reference to chains that are explicitly
created by RTM_NEWCHAIN message.

The test result:

 1..1
 ok 1 c2b4 - soft lockup alarm will be not generated after delete the prio 0
  filter of the chain

This is a follow up to commit c9a82bec ("net/sched: cls_api: Fix lockup on flushing explicitly created chain").
Signed-off-by: Mingshuai Ren <renmingshuai@huawei.com>
Acked-by: Pedro Tammela <pctammela@mojatatu.com>
Acked-by: Victor Nogueira <victor@mojatatu.com>
Link: https://lore.kernel.org/r/20230620014939.2034054-1-renmingshuai@huawei.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

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

+[
+    {
+        "id": "c2b4",
+        "name": "soft lockup alarm will be not generated after delete the prio 0 filter of the chain",
+        "category": [
+            "filter",
+            "chain"
+        ],
+        "setup": [
+            "$IP link add dev $DUMMY type dummy || /bin/true",
+            "$TC qdisc add dev $DUMMY root handle 1: htb default 1",
+            "$TC chain add dev $DUMMY",
+            "$TC filter del dev $DUMMY chain 0 parent 1: prio 0"
+        ],
+        "cmdUnderTest": "$TC filter add dev $DUMMY chain 0 parent 1:",
+        "expExitCode": "2",
+        "verifyCmd": "$TC chain ls dev $DUMMY",
+        "matchPattern": "chain parent 1: chain 0",
+        "matchCount": "1",
+        "teardown": [
+            "$TC qdisc del dev $DUMMY root handle 1: htb default 1",
+            "$IP link del dev $DUMMY type dummy"
+        ]
+    }
+]