Fix wait_for_debug_log acting as a noop

Summary:
wait_for_debug_log is not called using a with statement so the yield returns
the rest of the function as a generator that is never run.

Test Plan:
Wrap the "Avalanche invalidated block {tip_to_park}" check with:

with node.assert_debug_log([f"Avalanche invalidated block {tip_to_park}"]):

This fails!

ninja && ./test/functional/test_runner.py abc_p2p_avalanche_voting

Apply this patch and see that it now succeeds as expected.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: PiRK, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13172