chancloser: fix flake by registering subscriber first

Fixes another flake in the unit-race test: Sometimes we miss startup
events if there's a high CPU load (in CI for example).
To avoid that, we register our subscriber before starting the state
machine.

lnwallet/chancloser/rbf_coop_test.go

@@ -825,10 +825,13 @@ func newRbfCloserTestHarness(t *testing.T,
 	).Return(nil)
 
 	chanCloser := protofsm.NewStateMachine(protoCfg)
-	chanCloser.Start(ctx)
 
+	// We register our subscriber before starting the state machine, to make
+	// sure we don't miss any events.
 	harness.stateSub = chanCloser.RegisterStateEvents()
 
+	chanCloser.Start(ctx)
+
 	harness.chanCloser = &chanCloser
 
 	return harness