[EventEngine] Set DSCP on listener socket

Prepare the listener socket with the configured DSCP value to mark
outgoing packets correctly. This is already done correctly for clients.

During implementation and delivery of the DSCP feature both experiments
"event_engine_listener" and "event_engine_client" were default off.
Unfortunately the "event_engine_listener" experiment was not enabled correctly
during verification, which resulted in that the iomgr-path was verified instead.
This commit adds the missing action.

src/core/lib/event_engine/posix_engine/posix_engine_listener_utils.cc

@@ -169,6 +169,7 @@ absl::Status PrepareSocket(const PosixTcpOptions& options,
       !ResolvedAddressIsVSock(socket.addr)) {
     GRPC_RETURN_IF_ERROR(socket.sock.SetSocketLowLatency(1));
     GRPC_RETURN_IF_ERROR(socket.sock.SetSocketReuseAddr(1));
+    GRPC_RETURN_IF_ERROR(socket.sock.SetSocketDscp(options.dscp));
     socket.sock.TrySetSocketTcpUserTimeout(options, false);
   }
   GRPC_RETURN_IF_ERROR(socket.sock.SetSocketNoSigpipeIfPossible());