htlcswitch/test_utils: send htlcs before waiting for result

This commit is preparation for the test added in the subsequent commit.
We modify makeHoldPayment to return any failures direectly when trying
to add an HTLC to the switch. This lets us know that the HTLC was indeed
sent without failure when the method returns.

htlcswitch/test_utils.go

@@ -1345,15 +1345,13 @@ func (n *twoHopNetwork) makeHoldPayment(sendingPeer, receivingPeer lnpeer.Peer,
 	}
 
 	// Send payment and expose err channel.
-	go func() {
-		err := sender.htlcSwitch.SendHTLC(
-			firstHop, pid, htlc,
-		)
-		if err != nil {
-			paymentErr <- err
-			return
-		}
+	err = sender.htlcSwitch.SendHTLC(firstHop, pid, htlc)
+	if err != nil {
+		paymentErr <- err
+		return paymentErr
+	}
 
+	go func() {
 		resultChan, err := sender.htlcSwitch.GetPaymentResult(
 			pid, rhash, newMockDeobfuscator(),
 		)