Use constant for buffer size in OM functional_tests.rs

In 3c3e93e we ran rustfmt blindly
on `lightning/src/onion_message/functional_tests.rs`. This left
the file with a handful of warts that really should have been
cleaned up as we went.

Here we take the opportunity to clean up the `peer_buffer_full`
test which used a constant for the number of messages we can push
before the buffer fills, rather than calculating the amount based
on the buffer size constant, also removing a comment that got moved
by rustfmt into a place where it was no longer understandable.

Author: TheBlueMatt

lightning/src/onion_message/functional_tests.rs

@@ -765,8 +765,7 @@ fn peer_buffer_full() {
 	let destination = Destination::Node(nodes[1].node_id);
 	let instructions = MessageSendInstructions::WithoutReplyPath { destination };
 
-	for _ in 0..188 {
-		// Based on MAX_PER_PEER_BUFFER_SIZE in OnionMessenger
+	for _ in 0..super::messenger::MAX_PER_PEER_BUFFER_SIZE / 1401 + 1 {
 		nodes[0].messenger.send_onion_message(test_msg.clone(), instructions.clone()).unwrap();
 	}
 	let err = nodes[0].messenger.send_onion_message(test_msg, instructions.clone()).unwrap_err();

lightning/src/onion_message/messenger.rs

@@ -1661,11 +1661,12 @@ where
 	}
 }
 
+const MAX_TOTAL_BUFFER_SIZE: usize = (1 << 20) * 128;
+pub(super) const MAX_PER_PEER_BUFFER_SIZE: usize = (1 << 10) * 256;
+
 fn outbound_buffer_full(
 	peer_node_id: &PublicKey, buffer: &HashMap<PublicKey, OnionMessageRecipient>,
 ) -> bool {
-	const MAX_TOTAL_BUFFER_SIZE: usize = (1 << 20) * 128;
-	const MAX_PER_PEER_BUFFER_SIZE: usize = (1 << 10) * 256;
 	let mut total_buffered_bytes = 0;
 	let mut peer_buffered_bytes = 0;
 	for (pk, peer_buf) in buffer {