[tests] improve logging in feature_csv_activation.py

Summary:
Partial backport of core PR11817 (commit 2e511d5)
https://github.com/bitcoin/bitcoin/pull/11817/commits/2e511d54243e360c013079bc8f676934ef450ae3

Test Plan:

./test/functional/test_runner.py feature_csv_activation

Reviewers: #bitcoin_abc, deadalnix, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Maniphest Tasks: T669

Differential Revision: https://reviews.bitcoinabc.org/D3558