[test] Prevent feature_block spurious timeout

Summary:
There is no point using a shorter than default timeout, especially on a test that already takes a long time to complete.
This should prevent the occasional failures due to the timeout being just a bit too short.

Test Plan:

./test/functional/test_runner.py feature_block

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14154