Add automated testing

[mcopik]

We need an online testing service, as the offline plugins for serverless are not sufficient, e.g., Python triggers for DynamoDB streams do not work.

• [x] Research serverless CI/CD systems
• [x] Create a GH action creating a test deployment
• [x] Execute test suite
• [x] Clean up environment

[mcopik]

Finished, the CI is working.