test: capture loader logs, stop test port clashes

[catinspace-au]

A failing integration test printed only the ClickHouse container's stderr. The orchestrator runs inside the test process and nothing installed a tracing subscriber.

• tests/common/test_logging.rs installs a tracing-subscriber fmt layer with the test writer, once per process. TestInfrastructure::new calls it
• default filter is warn,dfe_loader=debug,scalo=info and RUST_LOG replaces it
• passing tests print nothing
• tracing-subscriber is a dev-dependency, the 0.3.23 scalo already resolves
• grpc_outage start() logs the error when the loader exits. It was thrown away
• free_port() claims each port with a marker socket held until the process exits

The last two go together. Test processes started together have consecutive pids, so free_port() searched from consecutive ports and probed the same listener port free for two of them. The second loader failed to bind, nothing logged it, and its client pushed into the first loader's table. That fits two grpc_outage failures that showed together in one run: "rows landed twice: {0}" in a_dlq_refusing_past_the_hold_holds_only_the_rows_it_must_take and "the first record never landed" in a_push_is_answered_only_once_its_row_is_in_clickhouse. a_port_is_handed_out_once fails on the old allocator and passes on the new one.

Done when a failing integration test shows the loader's own log lines and no two test processes are handed the same port.