test: remove unused PayInvoice mock path

Loop no longer pays invoices through LightningClient.PayInvoice.
Production payment paths use the router client wrappers. lndclient now
also implements PayInvoice on top of router payment APIs, so the old
SendPaymentChannel mock path is no longer exercised.

Remove the dead PayInvoice mock, its SendPaymentChannel plumbing, and
the loop-in resume assertion that watched that channel.

test/lightning_client_mock.go

@@ -29,21 +29,6 @@ type mockLightningClient struct {
 	wg  sync.WaitGroup
 }
 
-// PayInvoice pays an invoice.
-func (h *mockLightningClient) PayInvoice(ctx context.Context, invoice string,
-	maxFee btcutil.Amount,
-	outgoingChannel *uint64) chan lndclient.PaymentResult {
-
-	done := make(chan lndclient.PaymentResult, 1)
-
-	h.lnd.SendPaymentChannel <- PaymentChannelMessage{
-		PaymentRequest: invoice,
-		Done:           done,
-	}
-
-	return done
-}
-
 // DecodePaymentRequest returns a non-nil payment request.
 func (h *mockLightningClient) DecodePaymentRequest(_ context.Context,
 	_ string) (*lndclient.PaymentRequest, error) {

test/lnd_services_mock.go

@@ -48,7 +48,6 @@ func NewMockLnd() *LndMockServices {
 			ChainParams:   &chaincfg.TestNet3Params,
 			Versioner:     versioner,
 		},
-		SendPaymentChannel:           make(chan PaymentChannelMessage),
 		ConfChannel:                  make(chan *chainntnfs.TxConfirmation),
 		RegisterConfChannel:          make(chan *ConfRegistration),
 		RegisterSpendChannel:         make(chan *SpendRegistration),
@@ -95,12 +94,6 @@ func NewMockLnd() *LndMockServices {
 	return &lnd
 }
 
-// PaymentChannelMessage is the data that passed through SendPaymentChannel.
-type PaymentChannelMessage struct {
-	PaymentRequest string
-	Done           chan lndclient.PaymentResult
-}
-
 // TrackPaymentMessage is the data that passed through TrackPaymentChannel.
 type TrackPaymentMessage struct {
 	Hash lntypes.Hash
@@ -138,7 +131,6 @@ type PublishHandler func(ctx context.Context, tx *wire.MsgTx,
 type LndMockServices struct {
 	lndclient.LndServices
 
-	SendPaymentChannel   chan PaymentChannelMessage
 	SpendChannel         chan *chainntnfs.SpendDetail
 	TxPublishChannel     chan *wire.MsgTx
 	SendOutputsChannel   chan wire.MsgTx
@@ -221,12 +213,6 @@ func (s *LndMockServices) AddTx(tx *wire.MsgTx) {
 // IsDone checks whether all channels have been fully emptied. If not this may
 // indicate unexpected behaviour of the code under test.
 func (s *LndMockServices) IsDone() error {
-	select {
-	case <-s.SendPaymentChannel:
-		return errors.New("SendPaymentChannel not empty")
-	default:
-	}
-
 	select {
 	case <-s.SpendChannel:
 		return errors.New("SpendChannel not empty")