itest: add testSendDirectPayment

Adds a new test to check direct payments between two nodes. The fee rate
is tuned to 1 sat/vb to catch edge cases.

itest/list_on_test.go

@@ -177,6 +177,10 @@ var allTestCases = []*lntest.TestCase{
 		Name:     "list outgoing payments",
 		TestFunc: testListPayments,
 	},
+	{
+		Name:     "send direct payment",
+		TestFunc: testSendDirectPayment,
+	},
 	{
 		Name:     "immediate payment after channel opened",
 		TestFunc: testPaymentFollowingChannelOpen,