Use shared input's txOut in shouldSignFirst (#724)

When deciding who must send `tx_signatures` first for a splice, we
compute how much each node contributes to the splice transaction by
adding the amount of the `tx_add_input` they have sent.

For the shared input (current channel output), instead of using the
`txOut` amount, we previously summed the balance of each node in the
channel. The result is the same when there are no pending HTLCs, but
when there are pending HTLCs, it underestimates the amount of this
input.

This is hard to reproduce (and test) because it requires both nodes to
contribute to the splice while having large pending HTLCs that tilt the
balance in favor of the wrong side.

src/commonMain/kotlin/fr/acinq/lightning/channel/InteractiveTx.kt

@@ -1147,7 +1147,7 @@ data class InteractiveTxSigningSession(
         }
 
         fun shouldSignFirst(isInitiator: Boolean, channelParams: ChannelParams, tx: SharedTransaction): Boolean {
-            val sharedAmountIn = tx.sharedInput?.let { it.localAmount + it.remoteAmount } ?: 0.msat
+            val sharedAmountIn = tx.sharedInput?.txOut?.amount?.toMilliSatoshi() ?: 0.msat
             val localAmountIn = tx.localInputs.map { it.txOut.amount }.sum().toMilliSatoshi() + if (isInitiator) sharedAmountIn else 0.msat
             val remoteAmountIn = tx.remoteInputs.map { it.txOut.amount }.sum().toMilliSatoshi() + if (isInitiator) 0.msat else sharedAmountIn
             return if (localAmountIn == remoteAmountIn) {