test: fix timeout decrease in feature_assumevalid

Summary:
The timeout condition didn't work at all in assert_blockchain_height

This is a backport of core#21084

Test Plan: test/functional/test_runner.py feature_assumevalid

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10896