chancloser: stabilize remote RBF coop tests

[starius]

Change Description

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.

Steps to Test

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).

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

4. The panic reproduces immediately.

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