[tests] Remove nested loops from feature_csv_activation.py

Summary:
Makes the test a lot clearer.

Partial backport of core PR11817 (commit 0842edf)
https://github.com/bitcoin/bitcoin/pull/11817/commits/0842edf9ee7d659408e48b8897ac4a272e48b716

Depends on D3558

Test Plan:

./test/functional/test_runner.py feature_csv_activation

Reviewers: #bitcoin_abc, deadalnix, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Subscribers: markblundeberg

Maniphest Tasks: T669

Differential Revision: https://reviews.bitcoinabc.org/D3559