Fetch latest commitment txn via monitor, not channel in test

Eventually, we want to remove the Channel's copy of its own
ChannelMonitor, reducing memory footprint and complexity of
ChannelManager greatly.

This removes the last uses of said ChannelMonitor for latest
local commitment transactions (though it is still used for
would_broadcast_at_height(), which is the last remaining use).

Author: TheBlueMatt

lightning/src/ln/functional_test_utils.rs

@@ -259,7 +259,17 @@ macro_rules! get_feerate {
 
 macro_rules! get_local_commitment_txn {
 	($node: expr, $channel_id: expr) => {
-		$node.node.channel_state.lock().unwrap().by_id.get_mut(&$channel_id).unwrap().channel_monitor().get_latest_local_commitment_txn()
+		{
+			let mut monitors = $node.chan_monitor.simple_monitor.monitors.lock().unwrap();
+			let mut commitment_txn = None;
+			for (funding_txo, monitor) in monitors.iter_mut() {
+				if funding_txo.to_channel_id() == $channel_id {
+					commitment_txn = Some(monitor.get_latest_local_commitment_txn());
+					break;
+				}
+			}
+			commitment_txn.unwrap()
+		}
 	}
 }