Consistently pad the txs in feature_csv_activation

Summary: This will become a problem when we switch to MiniWallet for this test. Note that pad_tx() also rehashes the tx, so we can avoid doing it twice.

Test Plan:

./test/functional/test_runner.py feature_csv_activation

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16404