Add basic end-to-end splice tests

This adds a new test for both splice-in and splice-out in favor of
maintaining the existing test. Helpers have been added to DRY up a lot
of the logic necessary for driving the splice state machine forward.

Author: wpaulino

lightning/src/ln/channel.rs

@@ -6130,8 +6130,6 @@ where
 				holder_commitment_transaction_number,
 				self.counterparty_next_commitment_transaction_number,
 			);
-			// TODO(splicing) Forced error, as the use case is not complete
-			return Err(AbortReason::InternalError("Splicing not yet supported"));
 		} else {
 			self.assert_no_commitment_advancement(holder_commitment_transaction_number, "initial commitment_signed");
 			self.channel_state = ChannelState::FundingNegotiated(FundingNegotiatedFlags::new());

lightning/src/ln/funding.rs

@@ -17,6 +17,7 @@ use crate::prelude::Vec;
 use crate::sign::{P2TR_KEY_PATH_WITNESS_WEIGHT, P2WPKH_WITNESS_WEIGHT};
 
 /// The components of a splice's funding transaction that are contributed by one party.
+#[derive(Debug, Clone)]
 pub enum SpliceContribution {
 	/// When funds are added to a channel.
 	SpliceIn {
@@ -85,7 +86,7 @@ impl SpliceContribution {
 
 /// An input to contribute to a channel's funding transaction either when using the v2 channel
 /// establishment protocol or when splicing.
-#[derive(Clone)]
+#[derive(Debug, Clone)]
 pub struct FundingTxInput {
 	/// The unspent [`TxOut`] that the input spends.
 	///