[tests] style fixes in feature_pruning.py

Summary:
Minor style fixups. No functional change.

Partial backport of core PR15686 (commit 1c29ac4):
https://github.com/bitcoin/bitcoin/pull/15686/commits/1c29ac40fb1853df20572a389db70ce75d80ef93

Depends on D4560.

Test Plan:

./test/functional/test_runner.py feature_pruning

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4561