routing+itest: fix route blinding path selection bug

Here we fix a bug which would not include the selected incoming blinded
path chain from being included in the selected path set if it meets the
minimum length requirement. The appropriate unit test and itest is
updated to demonstrate the fix.

itest/lnd_route_blinding_test.go

@@ -1589,9 +1589,10 @@ func testPartiallySpecifiedBlindedPath(ht *lntest.HarnessTest) {
 		},
 	})
 
-	// Assert that it contains one path with 3 hops, with eve as the
-	// introduction node.
-	assertPathDetails(invoice, 3, eve.PubKey[:])
+	// Assert that it contains two paths: one 3 hop one starting at Eve and
+	// a 3 hop one starting at Dave (this one will be padded with a dummy
+	// hop) in order to keep all the paths equidistant.
+	assertPathDetails(invoice, 3, eve.PubKey[:], dave.PubKey[:])
 
 	// Check that Frank can pay the invoice.
 	ht.CompletePaymentRequests(frank, []string{invoice.PaymentRequest})