test: refactor: simplify p2p_tx_download.py and p2p_eviction by using MiniWallet

Summary:
Also, use the pre-mined chain of the test framework rather than
mining 100 blocks manually on each run.

This is a backport of core#26923
Depends on D18654

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18655