test: add assertion within integration tests for proper preimage after send

In this commit we modify the testSingleHopInvoice test to ensure that
we obtain the proper preimage from the SendPayment RPC upon successful
payment completion.

lnd_test.go

@@ -571,9 +571,16 @@ func testSingleHopInvoice(net *networkHarness, t *harnessTest) {
 	if err := sendStream.Send(sendReq); err != nil {
 		t.Fatalf("unable to send payment: %v", err)
 	}
-	if _, err := sendStream.Recv(); err != nil {
+
+	// Ensure we obtain the proper preimage in the response.
+	resp, err := sendStream.Recv()
+	if err != nil {
 		t.Fatalf("error when attempting recv: %v", err)
 	}
+	if !bytes.Equal(preimage, resp.PaymentPreimage) {
+		t.Fatalf("preimage mismatch: expected %v, got %v", preimage,
+			resp.PaymentPreimage)
+	}
 
 	// Bob's invoice should now be found and marked as settled.
 	// TODO(roasbeef): remove sleep after hooking into the to-be-written