[Chronik] Generate Chronik port for functional test

Summary:
Currently, chronik_block.py and chronik_serve.py sometimes fail, as their node ports overlap.

We add the same port allocation mechanism as for p2p_port and rpc_port to fix this.

Test Plan: ninja check-functional

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13294