routing/router: define PaymentAttemptDispatcher interface

The switch satisfies this interface, and makes it easy to mock the send
method from the router.

routing/mock_test.go

@@ -0,0 +1,31 @@
+package routing
+
+import (
+	"crypto/sha256"
+
+	"github.com/lightningnetwork/lnd/htlcswitch"
+	"github.com/lightningnetwork/lnd/lnwire"
+)
+
+type mockPaymentAttemptDispatcher struct {
+	onPayment func(firstHop lnwire.ShortChannelID) ([32]byte, error)
+}
+
+var _ PaymentAttemptDispatcher = (*mockPaymentAttemptDispatcher)(nil)
+
+func (m *mockPaymentAttemptDispatcher) SendHTLC(firstHop lnwire.ShortChannelID,
+	_ *lnwire.UpdateAddHTLC,
+	_ htlcswitch.ErrorDecrypter) ([sha256.Size]byte, error) {
+
+	if m.onPayment != nil {
+		return m.onPayment(firstHop)
+	}
+
+	return [sha256.Size]byte{}, nil
+}
+
+func (m *mockPaymentAttemptDispatcher) setPaymentResult(
+	f func(firstHop lnwire.ShortChannelID) ([32]byte, error)) {
+
+	m.onPayment = f
+}