chancloser: stabilize remote RBF coop tests

CI started panicking in TestRbfChannelFlushingTransitions/early_offer /
TestRbfCloseClosingNegotiationRemote (see GitHub Actions run
https://github.com/lightningnetwork/lnd/actions/runs/19155841408/job/54756127218?pr=10352)
because the cached remote offer could fire before the test harness
registered its mock CloseSigner expectations. When that happened,
the mock complained that CreateCloseProposal was unexpected:

panic:
assert: mock: I don't know what to return because the method call was unexpected.

Fix this by letting assertSingleRemoteRbfIteration take a trigger
callback. Tests that rely on asynchronously replayed offers now install
their expectations first and then fire the event via the callback.
All other call sites pass nil, preserving their existing behavior.

Reproduction (on master)
------------------------
1. Modify lnwallet/chancloser/rbf_coop_test.go
   Add time.Sleep(10 * time.Millisecond) before the first call of
   closeHarness.assertSingleRemoteRbfIteration (in function
   TestRbfChannelFlushingTransitions).

2. go test ./lnwallet/chancloser -run TestRbfChannelFlushingTransitions/early_offer

3. The panic reproduces immediately.

Author: starius

lnwallet/chancloser/rbf_coop_test.go

@@ -684,7 +684,7 @@ func (r *rbfCloserTestHarness) assertSingleRbfIteration(
 func (r *rbfCloserTestHarness) assertSingleRemoteRbfIteration(
 	initEvent *OfferReceivedEvent, balanceAfterClose,
 	absoluteFee btcutil.Amount, sequence uint32, iteration bool,
-	sendInit bool) {
+	trigger func()) {
 
 	ctx := r.T.Context()
 
@@ -696,7 +696,9 @@ func (r *rbfCloserTestHarness) assertSingleRemoteRbfIteration(
 		absoluteFee, balanceAfterClose, false,
 	)
 
-	if sendInit {
+	if trigger != nil {
+		trigger()
+	} else {
 		r.chanCloser.SendEvent(ctx, initEvent)
 	}
 
@@ -1386,10 +1388,13 @@ func TestRbfChannelFlushingTransitions(t *testing.T) {
 		// Now we'll send in the channel flushed event, and assert that
 		// this triggers a remote RBF iteration (we process their early
 		// offer and send our sig).
-		closeHarness.chanCloser.SendEvent(ctx, &flushEvent)
 		closeHarness.assertSingleRemoteRbfIteration(
 			remoteOffer, absoluteFee, absoluteFee, sequence, true,
-			false,
+			func() {
+				closeHarness.chanCloser.SendEvent(
+					ctx, &flushEvent,
+				)
+			},
 		)
 	})
 
@@ -1857,7 +1862,7 @@ func TestRbfCloseClosingNegotiationRemote(t *testing.T) {
 		// sig.
 		closeHarness.assertSingleRemoteRbfIteration(
 			feeOffer, balanceAfterClose, absoluteFee, sequence,
-			false, true,
+			false, nil,
 		)
 
 		// Next, we'll receive an offer from the remote party, and drive
@@ -1867,7 +1872,7 @@ func TestRbfCloseClosingNegotiationRemote(t *testing.T) {
 		absoluteFee = feeOffer.SigMsg.FeeSatoshis
 		closeHarness.assertSingleRemoteRbfIteration(
 			feeOffer, balanceAfterClose, absoluteFee, sequence,
-			true, true,
+			true, nil,
 		)
 
 		closeHarness.assertNoStateTransitions()
@@ -1950,7 +1955,7 @@ func TestRbfCloseClosingNegotiationRemote(t *testing.T) {
 		// sig.
 		closeHarness.assertSingleRemoteRbfIteration(
 			feeOffer, balanceAfterClose, absoluteFee, sequence,
-			false, true,
+			false, nil,
 		)
 	})
 
@@ -2048,7 +2053,7 @@ func TestRbfCloseErr(t *testing.T) {
 		// sig.
 		closeHarness.assertSingleRemoteRbfIteration(
 			feeOffer, balanceAfterClose, absoluteFee, sequence,
-			true, true,
+			true, nil,
 		)
 	})