psbt: Indicate which psbt inputs were signed

lntest/mock/walletcontroller.go

@@ -204,8 +204,8 @@ func (w *WalletController) FundPsbt(*psbt.Packet, int32, chainfee.SatPerKWeight,
 }
 
 // SignPsbt currently does nothing.
-func (w *WalletController) SignPsbt(*psbt.Packet) error {
-	return nil
+func (w *WalletController) SignPsbt(*psbt.Packet) ([]uint32, error) {
+	return nil, nil
 }
 
 // FinalizePsbt currently does nothing.