test: use MiniWallet for p2p_blocksonly.py

Summary:

This test can now be run even with the wallet disabled.

Backport of core#21867.

Depends on D12436.

Test Plan:
With and without wallet:

./test/functional/test_runner.py p2p_blocksonly

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12437