Convert run_db_test integration tests to pytest

[tomuben]

Background

The integration-tests still use the old unittest framework. We should switch to pytest.
The CLI Test should then use mocks, and the existing tests should use the API only.

Acceptance Criteria

Convert following tests to pytest:

• test_run_db_test_builtin_languages.py
• test_run_db_test_docker_credentials.py
• test_run_db_test_docker_db.py
• test_run_db_test_docker_db_check_arguments.py
• test_run_db_test_docker_db_reuse.py
• test_run_db_test_docker_pass_through.py
• test_run_db_test_external_db.py
• create tests for CLI using API mocks for run-db-test