[chronik] Prevent potential port collision in chronik_serve

Summary: The port 23456 is in the range of the ports used by the test framework itself so there is a risk it collides with another test.

Test Plan:

./test/functional/test_runner.py chronik_serve

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15707