Similar to #26892, this PR simplies the functional tests p2p_tx_download.py and p2p_eviction.py by using MiniWallet in order to avoid manual low-level tx creation. For the latter, rather than mining 100 blocks manually, the pre-mined chain of the test framework is used.
These instances were found via $ git grep signrawtransactionwithkey ./test/functional. AFAICT, there are no other instances where MiniWallet could replace tx creation trivially.