test: Use MiniWallet in mempool_limit.py
Summary:
This PR enables running another non-wallet functional test even when the wallet is disabled thanks to the MiniWallet, i.e. it can be run even when bitcoin-core is compiled with --disable-wallet.
Backport of core#22543.
Depends on D16385.
Test Plan:
With no wallet:
./test/functional/test_runner.py mempool_limit.py
Reviewers: #bitcoin_abc, roqqit
Reviewed By: roqqit
Subscribers: roqqit
Differential Revision: https://reviews.bitcoinabc.org/D16386