multi: send MPP payment to blinded path

Make various sender side adjustments so that a sender is able to send an
MP payment to a single blinded path without actually including an MPP
record in the payment.

itest/list_on_test.go

@@ -586,6 +586,10 @@ var allTestCases = []*lntest.TestCase{
 		Name:     "on chain to blinded",
 		TestFunc: testErrorHandlingOnChainFailure,
 	},
+	{
+		Name:     "mpp to single blinded path",
+		TestFunc: testMPPToSingleBlindedPath,
+	},
 	{
 		Name:     "removetx",
 		TestFunc: testRemoveTx,