Test acceptor contributions in dual-funding functional tests

We can now run through the case where the acceptor contributes to an
inbound channel, with either more value in inputs, or less value, leading
to a different `tx_signatures` exchange order.

We also cannot use dummy P2WPKH funding inputs and witnesses anymore as
`funding_transaction_signed` internally verifies signatures. Hence, we
create external keypairs that we can create outputs for and sign with.

Author: dunxen

lightning/src/ln/channel.rs

@@ -6314,6 +6314,19 @@ where
 		}
 	}
 
+	#[cfg(all(test))]
+	pub fn get_initial_counterparty_commitment_signatures_for_test<L: Deref>(
+		&mut self, funding: &mut FundingScope, logger: &L,
+		counterparty_next_commitment_point_override: PublicKey,
+	) -> Option<(Signature, Vec<Signature>)>
+	where
+		SP::Target: SignerProvider,
+		L::Target: Logger,
+	{
+		self.counterparty_next_commitment_point = Some(counterparty_next_commitment_point_override);
+		self.get_initial_counterparty_commitment_signatures(funding, logger)
+	}
+
 	fn check_funding_meets_minimum_depth(&self, funding: &FundingScope, height: u32) -> bool {
 		let minimum_depth = self
 			.minimum_depth(funding)