This PR enables one more of the non-wallet functional tests (mining_prioritisetransaction.py) to be run even with the Bitcoin Code wallet by using the MiniWallet instead, as proposed in #20078. Note that the adapted helper function create_lots_of_big_transactions is currently only used in this test, i.e. there was no need to change any others.
Backport of core#24839.
Note that the test is slightly modified due to D1587, as gettransaction cannot be used anymore.