lntest: add new wait for conf helper method to ChainNotifier

diff --git a/lntest/harness_assertion.go b/lntest/harness_assertion.go
index 9ae4251..8c60a27 100644
--- a/lntest/harness_assertion.go
+++ b/lntest/harness_assertion.go
@@ -18,6 +18,7 @@ import (
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
+	"github.com/davecgh/go-spew/spew"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/lnrpc"
 	"github.com/lightningnetwork/lnd/lnrpc/invoicesrpc"
@@ -552,8 +553,10 @@ func (h HarnessTest) WaitForChannelCloseEvent(
 	require.NoError(h, err)
 
 	resp, ok := event.Update.(*lnrpc.CloseStatusUpdate_ChanClose)
-	require.Truef(h, ok, "expected channel close update, instead got %v",
-		event.Update)
+	require.Truef(
+		h, ok, "expected channel close update, instead got %T: %v",
+		event.Update, spew.Sdump(event.Update),
+	)
 
 	txid, err := chainhash.NewHash(resp.ChanClose.ClosingTxid)
 	require.NoErrorf(h, err, "wrong format found in closing txid: %v",
diff --git a/lntest/mock/chainnotifier.go b/lntest/mock/chainnotifier.go
index ddce8de..9a9e125 100644
--- a/lntest/mock/chainnotifier.go
+++ b/lntest/mock/chainnotifier.go
@@ -1,6 +1,9 @@
 package mock
 
 import (
+	"testing"
+	"time"
+
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/lightningnetwork/lnd/chainntnfs"
@@ -8,9 +11,10 @@ import (
 
 // ChainNotifier is a mock implementation of the ChainNotifier interface.
 type ChainNotifier struct {
-	SpendChan chan *chainntnfs.SpendDetail
-	EpochChan chan *chainntnfs.BlockEpoch
-	ConfChan  chan *chainntnfs.TxConfirmation
+	SpendChan      chan *chainntnfs.SpendDetail
+	EpochChan      chan *chainntnfs.BlockEpoch
+	ConfChan       chan *chainntnfs.TxConfirmation
+	ConfRegistered chan struct{}
 }
 
 // RegisterConfirmationsNtfn returns a ConfirmationEvent that contains a channel
@@ -19,6 +23,14 @@ func (c *ChainNotifier) RegisterConfirmationsNtfn(txid *chainhash.Hash,
 	pkScript []byte, numConfs, heightHint uint32,
 	opts ...chainntnfs.NotifierOption) (*chainntnfs.ConfirmationEvent, error) {
 
+	// Signal that a confirmation registration occurred.
+	if c.ConfRegistered != nil {
+		select {
+		case c.ConfRegistered <- struct{}{}:
+		default:
+		}
+	}
+
 	return &chainntnfs.ConfirmationEvent{
 		Confirmed: c.ConfChan,
 		Cancel:    func() {},
@@ -61,3 +73,25 @@ func (c *ChainNotifier) Started() bool {
 func (c *ChainNotifier) Stop() error {
 	return nil
 }
+
+// WaitForConfRegistrationAndSend waits for a confirmation registration to
+// occur and then sends a confirmation notification. This is a helper function
+// for tests that need to ensure the chain watcher has registered for
+// confirmations before sending the confirmation.
+func (c *ChainNotifier) WaitForConfRegistrationAndSend(t *testing.T) {
+	t.Helper()
+
+	// Wait for the chain watcher to register for confirmations.
+	select {
+	case <-c.ConfRegistered:
+	case <-time.After(time.Second * 2):
+		t.Fatalf("timeout waiting for conf registration")
+	}
+
+	// Send the confirmation to satisfy the confirmation requirement.
+	select {
+	case c.ConfChan <- &chainntnfs.TxConfirmation{}:
+	case <-time.After(time.Second * 1):
+		t.Fatalf("unable to send confirmation")
+	}
+}