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.

ninja check-functional