input: pass input.Signature to multisig spend

Modifies SpendMultiSig to accept input.Signature, so that we can
ultimately assert the size of multisig witnesses.

lnwallet/chanvalidate/validate_test.go

@@ -98,7 +98,7 @@ func newChannelTestCtx(chanSize int64) (*channelTestCtx, error) {
 	}
 
 	sigHashes := txscript.NewTxSigHashes(commitTx)
-	aliceSig, err := txscript.RawTxInWitnessSignature(
+	aliceSigRaw, err := txscript.RawTxInWitnessSignature(
 		commitTx, sigHashes, 0, chanSize,
 		multiSigScript, txscript.SigHashAll, alicePriv,
 	)
@@ -106,7 +106,14 @@ func newChannelTestCtx(chanSize int64) (*channelTestCtx, error) {
 		return nil, err
 	}
 
-	bobSig, err := txscript.RawTxInWitnessSignature(
+	aliceSig, err := btcec.ParseDERSignature(
+		aliceSigRaw, btcec.S256(),
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	bobSigRaw, err := txscript.RawTxInWitnessSignature(
 		commitTx, sigHashes, 0, chanSize,
 		multiSigScript, txscript.SigHashAll, bobPriv,
 	)
@@ -114,6 +121,13 @@ func newChannelTestCtx(chanSize int64) (*channelTestCtx, error) {
 		return nil, err
 	}
 
+	bobSig, err := btcec.ParseDERSignature(
+		bobSigRaw, btcec.S256(),
+	)
+	if err != nil {
+		return nil, err
+	}
+
 	commitTx.TxIn[0].Witness = input.SpendMultiSig(
 		multiSigScript, alicePub.SerializeCompressed(), aliceSig,
 		bobPub.SerializeCompressed(), bobSig,