Run rpc_generateblock.py test even when the wallet is disabled, as discussed in #20078.
This PR picked up from where #20808 left off, started by @nginocchio. Since then, there have been many improvements to MiniWallet, making this PR more straightforward.
L23 makes use of MiniWallet.rescan_utxos() to add the pre-mined block utxos (when self.setup_clean_chain is not set to True), rather than generating new blocks during the test.