message: test: MiniWallet: add `send_to` method to create arbitrary txouts
  With this new method, outputs to an arbitrary scriptPubKey/amount can
  be created. Note that the implementation was already present in the
  test feature_rbf.py and is just moved to the MiniWallet interface, in
  order to enable other tests to also use it.
diff: @@ -146,6 +146,25 @@ class MiniWallet:
  self.sendrawtransaction(from_node=kwargs['from_node'], tx_hex=tx['hex'])
  return tx
  + def send_to(self, *, from_node, scriptPubKey, amount, fee=1000):
  + """
  + Create and send a tx with an output to a given scriptPubKey/amount,
  + plus a change output to our internal address. To keep things simple, a
  + fixed fee given in Satoshi is used.
  +
  + Note that this method fails if there is no single internal utxo
  + available that can cover the cost for the amount and the fixed fee
  + (the utxo with the largest value is taken).
  +
  + Returns a tuple (txid, n) referring to the created external utxo outpoint.
  + """
  + tx = self.create_self_transfer(from_node=from_node, fee_rate=0, mempool_valid=False)['tx']
  + assert_greater_than_or_equal(tx.vout[0].nValue, amount + fee)
  + tx.vout[0].nValue -= (amount + fee) # change output -> MiniWallet
  + tx.vout.append(CTxOut(amount, scriptPubKey)) # arbitrary output -> to be returned
  + txid = self.sendrawtransaction(from_node=from_node, tx_hex=tx.serialize().hex())
  + return txid, 1
  +
  def create_self_transfer(self, *, fee_rate=Decimal("0.003"), from_node, utxo_to_spend=None, mempool_valid=True, locktime=0, sequence=0):
  """Create and return a tx with the specified fee_rate. Fee may be exact or at most one satoshi higher than needed."""
  self._utxos = sorted(self._utxos, key=lambda k: k['value'])
  