lnwallet/chancloser: fix flake in TestRbfChannelFlushingTransitions/early_offer

In this commit, we fix a flake in the
`TestRbfChannelFlushingTransitions/early_offer` test. The fix is simple:
this is actually an "iteration", as we have a self transition to the
ChannelNegotiation state first. We also don't need to send the
remoteOffer, so we set `sendInit` to false. The offer still needs to be
passed in to ensure that the assertions work however.

lnwallet/chancloser/rbf_coop_test.go

@@ -1353,8 +1353,8 @@ func TestRbfChannelFlushingTransitions(t *testing.T) {
 		// offer and send our sig).
 		closeHarness.chanCloser.SendEvent(ctx, &flushEvent)
 		closeHarness.assertSingleRemoteRbfIteration(
-			remoteOffer, absoluteFee, absoluteFee, sequence, false,
-			true,
+			remoteOffer, absoluteFee, absoluteFee, sequence, true,
+			false,
 		)
 	})