test: MiniWallet: more deterministic coin selection for coinbase UTXOs (oldest first)

The coin selection strategy for MiniWallet is quite straight-forward: simply
pick a single UTXO with the largest value:

self._utxos = sorted(self._utxos, key=lambda k: k['value'])
utxo_to_spend = utxo_to_spend or self._utxos.pop()

If there are several candidates with the same value, however, it is not clear
which one is taken.  This can be particularly problematic for coinbase outputs
with fixed block subsidy, since spending could lead to a
'bad-txns-premature-spend-of-coinbase' reject if an UTXO from a too-recent
block is picked.  Introduce block height as second criteria (saved in
self._utxos in the methods generate(...) and rescan_utxos(...)), in order to
avoid potential issues with coinbases that are not matured yet.

test/functional/test_framework/wallet.py

@@ -88,13 +88,13 @@ class MiniWallet:
         res = self._test_node.scantxoutset(action="start", scanobjects=[self.get_descriptor()])
         assert_equal(True, res['success'])
         for utxo in res['unspents']:
-            self._utxos.append({'txid': utxo['txid'], 'vout': utxo['vout'], 'value': utxo['amount']})
+            self._utxos.append({'txid': utxo['txid'], 'vout': utxo['vout'], 'value': utxo['amount'], 'height': utxo['height']})
 
     def scan_tx(self, tx):
         """Scan the tx for self._scriptPubKey outputs and add them to self._utxos"""
         for out in tx['vout']:
             if out['scriptPubKey']['hex'] == self._scriptPubKey.hex():
-                self._utxos.append({'txid': tx['txid'], 'vout': out['n'], 'value': out['value']})
+                self._utxos.append({'txid': tx['txid'], 'vout': out['n'], 'value': out['value'], 'height': 0})
 
     def sign_tx(self, tx, fixed_length=True):
         """Sign tx that has been created by MiniWallet in P2PK mode"""
@@ -115,8 +115,9 @@ class MiniWallet:
         """Generate blocks with coinbase outputs to the internal address, and append the outputs to the internal list"""
         blocks = self._test_node.generatetodescriptor(num_blocks, self.get_descriptor(), **kwargs)
         for b in blocks:
-            cb_tx = self._test_node.getblock(blockhash=b, verbosity=2)['tx'][0]
-            self._utxos.append({'txid': cb_tx['txid'], 'vout': 0, 'value': cb_tx['vout'][0]['value']})
+            block_info = self._test_node.getblock(blockhash=b, verbosity=2)
+            cb_tx = block_info['tx'][0]
+            self._utxos.append({'txid': cb_tx['txid'], 'vout': 0, 'value': cb_tx['vout'][0]['value'], 'height': block_info['height']})
         return blocks
 
     def get_descriptor(self):
@@ -170,7 +171,7 @@ class MiniWallet:
 
     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'])
+        self._utxos = sorted(self._utxos, key=lambda k: (k['value'], -k['height']))
         utxo_to_spend = utxo_to_spend or self._utxos.pop()  # Pick the largest utxo (if none provided) and hope it covers the fee
         if self._priv_key is None:
             vsize = Decimal(96)  # anyone-can-spend