[chronik] Improve coverage of avalanche event ws messages

Summary:
Add a new test that mimics the last part of chronik_ws.py (actually creating some duplicata but with a different intent) while using all the subscription methods at the same time. This makes sure we get the expected messages with all these subscription kinds.

The constants removal in chronik_ws.py are unused and found during the test review while building the new one and has been included here because it does not deserve its own diff and is somewhat related.

Depends on D18542.

Test Plan:

./test/functional/test_runner.py chronik_ws chronik_ws_avalanche

Reviewers: #bitcoin_abc, PiRK, tobias_ruck

Reviewed By: #bitcoin_abc, PiRK, tobias_ruck

Subscribers: tobias_ruck, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18546