cpu: TrafficGen sinks snoops without complaining

To be able to use the TrafficGen in a system with caches we need to
allow it to sink incoming snoop requests. By default the master port
panics, so silently ignore any snoops.

src/cpu/testers/traffic_gen/traffic_gen.hh

@@ -152,6 +152,12 @@ class TrafficGen : public MemObject
 
         bool recvTimingResp(PacketPtr pkt);
 
+        void recvTimingSnoopReq(PacketPtr pkt) { }
+
+        void recvFunctionalSnoop(PacketPtr pkt) { }
+
+        Tick recvAtomicSnoop(PacketPtr pkt) { return 0; }
+
       private:
 
         TrafficGen& trafficGen;