routing/test: close payment result channel on shutdown, mimicking switch

This commit updates our mock to more closely follow the behavior of the
switch for mocked calls to GetPaymentResult. As it stands, our tests
send a test-created error from the switch when we want to mock shutdown.
In reality, the switch will close its result channel, so we update this
test to follow that behavior. This matters for the commit that follows,
because we start checking the error our payments return. If we have an
error from the switch, our tests will fail with an error that we do
not encounter in practice.

routing/mock_test.go

@@ -155,10 +155,9 @@ func (m *mockPaymentSession) RequestRoute(_, _ lnwire.MilliSatoshi,
 }
 
 type mockPayer struct {
-	sendResult       chan error
-	paymentResultErr chan error
-	paymentResult    chan *htlcswitch.PaymentResult
-	quit             chan struct{}
+	sendResult    chan error
+	paymentResult chan *htlcswitch.PaymentResult
+	quit          chan struct{}
 }
 
 var _ PaymentAttemptDispatcher = (*mockPayer)(nil)
@@ -180,12 +179,16 @@ func (m *mockPayer) GetPaymentResult(paymentID uint64, _ lntypes.Hash,
 	_ htlcswitch.ErrorDecrypter) (<-chan *htlcswitch.PaymentResult, error) {
 
 	select {
-	case res := <-m.paymentResult:
+	case res, ok := <-m.paymentResult:
 		resChan := make(chan *htlcswitch.PaymentResult, 1)
-		resChan <- res
+		if !ok {
+			close(resChan)
+		} else {
+			resChan <- res
+		}
+
 		return resChan, nil
-	case err := <-m.paymentResultErr:
-		return nil, err
+
 	case <-m.quit:
 		return nil, fmt.Errorf("test quitting")
 	}