Reduce number of running containers in CI to one per run

## Observed

Each CI tests run creates a few ClickHouse containers per test directory. This is an unnecessary overhead that could be reduced to a single container per run.

## Expected behaviour

Run a single ClickHouse container per tests run.

Start by locating the CI test setup that creates ClickHouse containers for each test directory. Trace how containers are created and scoped during a test run, then verify that a CI run uses one ClickHouse container while all test directories still execute successfully.

Written by the indexing model from the issue text.