Move channel reserve tests out of functional_tests.rs into a new file

`functional_tests.rs` has gotten incredibly huge over the years, so
here we move some channel reserve tests into a new file.

Author: TheBlueMatt

lightning/src/ln/functional_test_utils.rs

@@ -16,7 +16,9 @@ use crate::chain::transaction::OutPoint;
 use crate::events::{ClaimedHTLC, ClosureReason, Event, HTLCHandlingFailureType, PaidBolt12Invoice, PathFailure, PaymentFailureReason, PaymentPurpose};
 use crate::events::bump_transaction::{BumpTransactionEvent, BumpTransactionEventHandler, Wallet, WalletSource};
 use crate::ln::types::ChannelId;
+use crate::types::features::ChannelTypeFeatures;
 use crate::types::payment::{PaymentPreimage, PaymentHash, PaymentSecret};
+use crate::ln::chan_utils::{commitment_tx_base_weight, COMMITMENT_TX_WEIGHT_PER_HTLC};
 use crate::ln::channelmanager::{AChannelManager, ChainParameters, ChannelManager, ChannelManagerReadArgs, RAACommitmentOrder, RecipientOnionFields, PaymentId, MIN_CLTV_EXPIRY_DELTA};
 use crate::types::features::InitFeatures;
 use crate::ln::msgs;
@@ -1104,6 +1106,10 @@ macro_rules! unwrap_send_err {
 	}
 }
 
+pub fn commit_tx_fee_msat(feerate: u32, num_htlcs: u64, channel_type_features: &ChannelTypeFeatures) -> u64 {
+	(commitment_tx_base_weight(channel_type_features) + num_htlcs * COMMITMENT_TX_WEIGHT_PER_HTLC) * feerate as u64 / 1000 * 1000
+}
+
 /// Check whether N channel monitor(s) have been added.
 pub fn check_added_monitors<CM: AChannelManager, H: NodeHolder<CM=CM>>(node: &H, count: usize) {
 	if let Some(chain_monitor) = node.chain_monitor() {