Vision Service: Add unit tests for the most important functionality

[lastzero]

As a developer, I want the project to have unit test coverage for the most important functionality, like request handling, so I can quickly check if my changes or new pull requests would break things, and the code is easy to refactor:

• Unit tests must follow our test automation guidelines and use a framework/library that can be easily integrated with CI tools.
• The unit test suite should not take a long time to run, so it can be run frequently. It should also be easy to configure, ideally requiring no configuration when running in a preconfigured Docker image.
• How to run the unit tests and some basic information on how to add tests must be documented in the README.md file file, or in a separate location with a link to it.