Don't rely on route-fetching rand in blinded_path_with_custom_tlv

`blinded_path_with_custom_tlv` indirectly relied on route CLTV
randomization when sending because nodes were at substantially
different block heights after setup. Instead we make sure all nodes
are at the same height which makes the test more robust.

lightning/src/ln/max_payment_path_len_tests.rs

@@ -255,6 +255,13 @@ fn blinded_path_with_custom_tlv() {
 	create_announced_chan_between_nodes(&nodes, 1, 2);
 	let chan_upd_2_3 = create_announced_chan_between_nodes_with_value(&nodes, 2, 3, 1_000_000, 0).0.contents;
 
+	// Ensure all nodes are at the same height
+	let node_max_height = nodes.iter().map(|node| node.blocks.lock().unwrap().len()).max().unwrap() as u32;
+	connect_blocks(&nodes[0], node_max_height - nodes[0].best_block_info().1);
+	connect_blocks(&nodes[1], node_max_height - nodes[1].best_block_info().1);
+	connect_blocks(&nodes[2], node_max_height - nodes[2].best_block_info().1);
+	connect_blocks(&nodes[3], node_max_height - nodes[3].best_block_info().1);
+
 	// Construct the route parameters for sending to nodes[3]'s blinded path.
 	let amt_msat = 100_000;
 	let (payment_preimage, payment_hash, payment_secret) = get_payment_preimage_hash(&nodes[3], Some(amt_msat), None);