Check that we aren't reading a second message in BOLT 12 retry test

`creates_and_pays_for_offer_with_retry` intends to check that we
re-send a BOLT 12 `invoice_request` in response to a
`message_received` call, but doesn't actually test that there were
no messages in the outbound buffer after the initial send, which we
do here.

Author: TheBlueMatt

lightning/src/ln/offers_tests.rs

@@ -1100,6 +1100,7 @@ fn creates_and_pays_for_offer_with_retry() {
 	expect_recent_payment!(bob, RecentPaymentDetails::AwaitingInvoice, payment_id);
 
 	let _lost_onion_message = bob.onion_messenger.next_onion_message_for_peer(alice_id).unwrap();
+	assert!(bob.onion_messenger.next_onion_message_for_peer(alice_id).is_none());
 
 	// Simulate a scenario where the original onion_message is lost before reaching Alice.
 	// Use handle_message_received to regenerate the message.