routing/result_interpretation: fix off-by-one for incoming failure

Previously we would not mark a success for the first hop if the fail
source index was 2. We also add a test to assert this behavior.

routing/result_interpretation.go

@@ -265,7 +265,7 @@ func (i *interpretedResult) processPaymentOutcomeIntermediate(
 
 		// All nodes up to the failing pair must have forwarded
 		// successfully.
-		if errorSourceIdx > 2 {
+		if errorSourceIdx > 1 {
 			i.successPairRange(route, 0, errorSourceIdx-2)
 		}
 	}