test: Fix feature_init intermittent issues

Summary:

The test doesn't work currently because the log might be finalized before wait_for_debug_log is started, in which case it will assume the log is empty and fail to detect any line.

Fix this by calling wait_for_debug_log first. Fixes core#24060.

This concludes backport of core#24192
https://github.com/bitcoin/bitcoin/pull/24192/commits/fa7b07571f24b6def6effdd4cc1b96c7507bf959

Depends on D12620

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12621