Prefer wait_until over polling with time.sleep

Summary:

• test: Use wait_until in tests where time was used for polling
• test: Use os.path.join consistently in feature_pruning tests

This is a backport of Core PR12553

Test Plan:

make check
./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4082