message: test: MiniWallet: Default fee_rate in send_self_transfer, Pass in utxo_to_spend
  Adds two new features to MiniWallet:
  * The fee rate is irrelevant sometimes, so just set an arbitrary default
  * The utxo to spend needs to be selected manually sometimes
diff: @@ -40,17 +40,17 @@ class MiniWallet:
  self._utxos.append({'txid': cb_tx['txid'], 'vout': 0, 'value': cb_tx['vout'][0]['value']})
  return blocks
  - def send_self_transfer(self, *, fee_rate, from_node):
  + def send_self_transfer(self, *, fee_rate=Decimal("0.003"), from_node, utxo_to_spend=None):
  """Create and send 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'])
  - largest_utxo = self._utxos.pop() # Pick the largest utxo and hope it covers the fee
  + utxo_to_spend = utxo_to_spend or self._utxos.pop() # Pick the largest utxo (if none provided) and hope it covers the fee
  vsize = Decimal(96)
  - send_value = satoshi_round(largest_utxo['value'] - fee_rate * (vsize / 1000))
  - fee = largest_utxo['value'] - send_value
  + send_value = satoshi_round(utxo_to_spend['value'] - fee_rate * (vsize / 1000))
  + fee = utxo_to_spend['value'] - send_value
  assert send_value > 0
  tx = CTransaction()
  - tx.vin = [CTxIn(COutPoint(int(largest_utxo['txid'], 16), largest_utxo['vout']))]
  + tx.vin = [CTxIn(COutPoint(int(utxo_to_spend['txid'], 16), utxo_to_spend['vout']))]
  tx.vout = [CTxOut(int(send_value * COIN), self._scriptPubKey)]
  tx.wit.vtxinwit = [CTxInWitness()]
  tx.wit.vtxinwit[0].scriptWitness.stack = [CScript([OP_TRUE])]
  