Enable in-container unit tests for mno_data_source_simulator

[cchwala]

Currently the mno_simulator tests cannot be run from within the docker container. This could/should be changed.

• Add a test-stage (or optional Dockerfile target) that copies tests/ and installs tests/requirements.txt.
• Keep the runtime image unchanged; use the test image for CI or run locally with docker compose run --rm mno_simulator pytest -m "not integration".
• Update CI to run tests using that test profile to ensure container-based test parity.