selftests: add a tc matchall test case

This is a follow up of the commit 0db6f8be ("net/sched: fix ->get
helper of the matchall cls").

To test it:
$ cd tools/testing/selftests/tc-testing
$ ln -s ../plugin-lib/nsPlugin.py plugins/20-nsPlugin.py
$ ./tdc.py -n -e 2638
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/tc-testing/tc-tests/filters/tests.json

@@ -18,6 +18,26 @@
             "$TC qdisc del dev $DEV1 ingress"
         ]
     },
+    {
+        "id": "2638",
+        "name": "Add matchall and try to get it",
+        "category": [
+            "filter",
+            "matchall"
+        ],
+        "setup": [
+            "$TC qdisc add dev $DEV1 clsact",
+            "$TC filter add dev $DEV1 protocol all pref 1 ingress handle 0x1234 matchall action ok"
+        ],
+        "cmdUnderTest": "$TC filter get dev $DEV1 protocol all pref 1 ingress handle 0x1234 matchall",
+        "expExitCode": "0",
+        "verifyCmd": "$TC filter show dev $DEV1 ingress",
+        "matchPattern": "filter protocol all pref 1 matchall chain 0 handle 0x1234",
+        "matchCount": "1",
+        "teardown": [
+            "$TC qdisc del dev $DEV1 clsact"
+        ]
+    },
     {
         "id": "d052",
         "name": "Add 1M filters with the same action",