[tests] fix block time in feature_pruning.py
Summary:
This is failing on travis because the mine_large_blocks() is creating blocks with too-old timestamps. This only becomes apparent when running the test on slow systems.
Backport of core PR15696:
https://github.com/bitcoin/bitcoin/pull/15696/files
Depends on D4561.
Test Plan:
./test/functional/test_runner.py feature_pruning
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D4563