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 having assertSingleRemoteRbfIteration take a sendEvent callback
that receives the context and initial offer, so tests can install expectations
first and then fire the event via SendEvent (or the early-offer test's custom
flush sender).
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.