cmd/devp2p/internal/ethtest: validate received txs, not the sent ones (#35170)

sendInvalidTxs's *eth.TransactionsPacket case iterated `txs` — the
locally-sent invalid transactions, every one of which is in `invalids`
by construction — instead of the transactions actually carried by the
received packet. As a result the loop returned "received bad tx" on the
very first TransactionsPacket the peer sent, regardless of its contents,
and never inspected what was really propagated.

Iterate msg.Items() (the decoded contents of the received packet) so the
"node must not propagate invalid txs" conformance check tests the real
condition instead of producing a false negative.

---------

Co-authored-by: Bosul Mun <bsbs8645@snu.ac.kr>

cmd/devp2p/internal/ethtest/transaction.go

@@ -155,7 +155,11 @@ func (s *Suite) sendInvalidTxs(t *utesting.T, txs []*types.Transaction) error {
 
 		switch msg := msg.(type) {
 		case *eth.TransactionsPacket:
-			for _, tx := range txs {
+			received, err := msg.Items()
+			if err != nil {
+				return fmt.Errorf("failed to decode received transactions: %w", err)
+			}
+			for _, tx := range received {
 				if _, ok := invalids[tx.Hash()]; ok {
 					return fmt.Errorf("received bad tx: %s", tx.Hash())
 				}