[Chronik] Add unix socket listener to test_runner.rs

Summary:
Depends on D17564

A Unix socket is used for communication between the parent (Rust) and child (Python) processes. In future iterations, the socket stream will be passed to a handler. The Unix socket is only checked in ipc.py if it's passed as an environment variable to the child process. Therefore, this change does not affect the current TypeScript Chronik tests.

Test Plan: cd modules/bitcoinsuite-chronik-client && cargo test -- --nocapture

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17565