test: use MiniWallet for mining_getblocktemplate_longpoll.py

Summary:
This test can now be run even with the Bitcoin ABC wallet disabled.

This is a backport of core#20159 [1/3]
https://github.com/bitcoin/bitcoin/pull/20159/commits/fddce7e199308d96e366d700dca982ef088ba98b

Test Plan: test/functional/test_runner.py mining_getblocktemplate_longpoll

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10517