Support overpayment in test util pass_along_path.

Useful for payments to blinded paths where rounding errors during
BlindedPayInfo aggregation may result in overpayment.

lightning/src/ln/functional_test_utils.rs

@@ -2407,6 +2407,9 @@ pub struct PassAlongPathArgs<'a, 'b, 'c, 'd> {
 	pub clear_recipient_events: bool,
 	pub expected_preimage: Option<PaymentPreimage>,
 	pub is_probe: bool,
+	// Allow overpayment to the recipient up to this amount. Useful for payments to blinded paths
+	// where rounding errors during `BlindedPayInfo` aggregation may result in overpayment.
+	pub overpay_limit: u64,
 }
 
 impl<'a, 'b, 'c, 'd> PassAlongPathArgs<'a, 'b, 'c, 'd> {
@@ -2417,7 +2420,7 @@ impl<'a, 'b, 'c, 'd> PassAlongPathArgs<'a, 'b, 'c, 'd> {
 		Self {
 			origin_node, expected_path, recv_value, payment_hash, payment_secret: None, event,
 			payment_claimable_expected: true, clear_recipient_events: true, expected_preimage: None,
-			is_probe: false,
+			is_probe: false, overpay_limit: 0,
 		}
 	}
 	pub fn clear_recipient_events(mut self, clear_recipient_events: bool) -> Self {
@@ -2441,13 +2444,17 @@ impl<'a, 'b, 'c, 'd> PassAlongPathArgs<'a, 'b, 'c, 'd> {
 		self.expected_preimage = Some(payment_preimage);
 		self
 	}
+	pub fn with_overpay_limit(mut self, overpay_limit: u64) -> Self {
+		self.overpay_limit =  overpay_limit;
+		self
+	}
 }
 
 pub fn do_pass_along_path<'a, 'b, 'c>(args: PassAlongPathArgs) -> Option<Event> {
 	let PassAlongPathArgs {
 		origin_node, expected_path, recv_value, payment_hash: our_payment_hash,
 		payment_secret: our_payment_secret, event: ev, payment_claimable_expected,
-		clear_recipient_events, expected_preimage, is_probe
+		clear_recipient_events, expected_preimage, is_probe, overpay_limit,
 	} = args;
 
 	let mut payment_event = SendEvent::from_event(ev);
@@ -2491,7 +2498,8 @@ pub fn do_pass_along_path<'a, 'b, 'c>(args: PassAlongPathArgs) -> Option<Event>
 								assert_eq!(our_payment_secret, onion_fields.as_ref().unwrap().payment_secret);
 							},
 						}
-						assert_eq!(*amount_msat, recv_value);
+						assert!(*amount_msat >= recv_value);
+						assert!(*amount_msat <= recv_value + overpay_limit);
 						assert!(node.node.list_channels().iter().any(|details| details.channel_id == via_channel_id.unwrap()));
 						assert!(node.node.list_channels().iter().any(|details| details.user_channel_id == via_user_channel_id.unwrap()));
 						assert!(claim_deadline.unwrap() > node.best_block_info().1);