Bump the timeout for the feature_cltv test

Summary: This test occasionally fails to mine the 1339 blocks in a row on the slow osx CI machine.

Test Plan:

./test/functional/test_runner.py feature_cltv

Reviewers: #bitcoin_abc, roqqit

Reviewed By: #bitcoin_abc, roqqit

Differential Revision: https://reviews.bitcoinabc.org/D17585