Test for extra locks held in `handle_error` unconditionally

Test for extra locks held in `handle_error` unconditionally

`handle_error` must be called without `per_peer_state` mutex or
`pending_events` mutex locks held or we may risk deadlocks.
Previously we checked this in debug builds in the error path, but
not in the success path.

As it turns out, `funding_transaction_generated`'s error path does
hold a `per_peer_state` lock, which we fix here as well as move the
tests to happen unconditionally.