allow Non-ECT markings when validating (#394)

* fix: Allow non-ECN marks when validating

Because I think the algorithm in RFC9000 says "probe with N marked
packets, then check the ACKs", which implies that an implementation
needs to stop marking after N to have a stable comparison value.

* Simplify

Author: larseggert

testcases.py

@@ -1113,11 +1113,7 @@ def _check_ecn_any(self, e) -> bool:
         return e[ECN.ECT0] != 0 or e[ECN.ECT1] != 0
 
     def _check_ecn_marks(self, e) -> bool:
-        return (
-            e[ECN.NONE] == 0
-            and e[ECN.CE] == 0
-            and ((e[ECN.ECT0] == 0) != (e[ECN.ECT1] == 0))
-        )
+        return e[ECN.CE] == 0 and self._check_ecn_any(e)
 
     def _check_ack_ecn(self, tr) -> bool:
         # NOTE: We only check whether the trace contains any ACK-ECN information, not whether it is valid