Test suites that use Testcontainers seem to be flaky

Various test that depend on Testcontainers seem to be flaky, GitHub Action run history shows - `WordCountDocExampleSpec`, `KafkaToSlickIntegrationSpec` , `CassandraProjectionTest`.

If there is ever time/will to stabilize this, one option would be to get rid of Testcontainers starting the services within tests and to move the services out to a Docker Compose that is started before running tests, and then to modify tests to work within these shared services.

Review GitHub Actions run history for WordCountDocExampleSpec, KafkaToSlickIntegrationSpec, and CassandraProjectionTest to identify the flaky failures. Then inspect how these tests start and use Testcontainers and evaluate the proposed Docker Compose setup. Done means the affected test suites run reliably against the shared services.

Written by the indexing model from the issue text.