Commit the OpenAPI generated tests into "./tests" and make them run successfully

Parent Epic: [EPIC] Minimal test suite for Engine API (#5 - closed)

• Move tests in engine-api/src/aura_engine_api/rest/test to engine-api/tests
• Make them runnable with make test
• Make them succeed locally
• Use SQLite for the CI/CD and make them succeed in the pipeline
• Update the README, how we should deal with generated API client implementation & test code in the future. When we implement a local OpenAPI client generation, such tests should be ideally already generated to engine-api/tests. Compare the ticket Generate API locally using "OpenAPI Generator" (#37 - closed)