[chronik] Fix websocket subscription in tests

Summary: The tests expect the websocket subscription to take effect immediately, however this doesn't hold as the subscription is send via the network and can take some time to be received/processed. This diff uses the log to make sure the subscription happened before returning.

Test Plan:

./test/functional/test_runner.py chronik_*

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: tobias_ruck, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15461