Onion messages: add fuzz testing

[valentinewallace]

Adds fuzz testing for onion messages.

[TheBlueMatt]

Test LGTM, giving this some CPU cycles now.

[TheBlueMatt]

Hmmm, I gave this a few weeks of CPU time and it wasn't able to find its way into a valid onion message decryption at all. This tells me we need to seed the fuzzer, like the way we do full_stack_target, at least, if not find a way to simplify the target here. I'm not sure quite how to simplify the target greatly, though, so maybe just seed it and move on.

[TheBlueMatt]

Specifically, the fuzzer did not fail with this patch:

diff --git a/lightning/src/onion_message/messenger.rs b/lightning/src/onion_message/messenger.rs
index 7eba3cdd..617ed434 100644
--- a/lightning/src/onion_message/messenger.rs
+++ b/lightning/src/onion_message/messenger.rs
@@ -211,11 +211,13 @@ impl<Signer: Sign, K: Deref, L: Deref> OnionMessenger<Signer, K, L>
                        Ok((Payload::Receive {
                                control_tlvs: ReceiveControlTlvs::Unblinded(ReceiveTlvs { path_id })
                        }, None)) => {
+panic!("1");
                                log_info!(self.logger, "Received an onion message with path_id: {:02x?}", path_id);
                        },
                        Ok((Payload::Forward(ForwardControlTlvs::Unblinded(ForwardTlvs {
                                next_node_id, next_blinding_override
                        })), Some((next_hop_hmac, new_packet_bytes)))) => {
+panic!("2");
                                // TODO: we need to check whether `next_node_id` is our node, in which case this is a dummy
                                // blinded hop and this onion message is destined for us. In this situation, we should keep
                                // unwrapping the onion layers to get to the final payload. Since we don't have the option

[valentinewallace]

I added some seeding but could add more onion message seeds if it helps.

Unsure if it would be good to add msg_targets for onion_message::Payload and/or BlindedRoute?

[valentinewallace]

Able to hit the obvious panics now. I put the seeds in test_no_onion_message_breakage -- not sure if they should be checked into git as well?