[avalanche] Improve the transaction voting test

Summary: Consistently use the MiniWallet to build transaction and the send_txs_and test function. This makes the tests simpler with less dependencies and easier to read. There is no change in behavior.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_transaction_voting

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16648