tc-testing: fix list handling

python lists don't have an 'add' method, but 'append'.

Fixes: 14e5175e9e ("tc-testing: introduce scapyPlugin for basic traffic")
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/tc-testing/plugin-lib/scapyPlugin.py

@@ -36,7 +36,7 @@ class SubPlugin(TdcPlugin):
         for k in scapy_keys:
             if k not in scapyinfo:
                 keyfail = True
-                missing_keys.add(k)
+                missing_keys.append(k)
         if keyfail:
             print('{}: Scapy block present in the test, but is missing info:'
                 .format(self.sub_class))