[chronik] Fix a race in chronik_ws

Summary: The test needs to drain the validation interface to avoid out of order messages, otherwise the tip finalization might not have been processed when the blocks are subscribed.

Test Plan:

./test/functional/test_runner.py chronik_ws

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15702