In this commit, we fix a flake in the
`TestRbfCloseClosingNegotiationLocal/send_offer_rbf_wrong_local_script`
test.
This flake can happen if the test shuts down _before_ the state machine
is actually able to process the sent event. In this case, the
expectations are triggered, and we find that the error isn't sent.
To resolve this, we create a new wrapper function that'll use a sync
channel send to assert that the error has been sent before we exit the
test.
Olaoluwa Osuntokun