[create-artifact-worker] acceptance tests for docker-compose Artifacts generated with "generate" endpoint

Acceptance criteria:

• Acceptance tests for /deployments/artifacts/generate endpoint, implemented in MEN-3847.
• Must test at least:
  • Creation with and without unpinned flag set
  • Invalid parameters
  • Invalid image name in docker-compose file, which will cause error code 5. Make sure this produces expected behavior from MEN-3847.