f use helper

Author: TheBlueMatt

lightning/src/ln/functional_tests.rs

@@ -1514,7 +1514,7 @@ fn test_chan_init_feerate_unaffordability() {
 	// HTLC.
 	let feerate_per_kw = 253;
 	let mut push_amt = 100_000_000;
-	push_amt -= feerate_per_kw as u64 * (COMMITMENT_TX_BASE_WEIGHT + 4 * COMMITMENT_TX_WEIGHT_PER_HTLC) / 1000 * 1000;
+	push_amt -= commit_tx_fee_msat(feerate_per_kw, 4);
 	assert_eq!(nodes[0].node.create_channel(nodes[1].node.get_our_node_id(), 100_000, push_amt + 1, 42, None).unwrap_err(),
 		APIError::APIMisuseError { err: "Funding amount (356) can't even pay fee for initial commitment transaction fee of 357.".to_string() });