Set up basic pytest unit tests for Ray service

Description

Currently, NDIF has very few tests written for it, with the barrier to entry being the lack of an existing testing framework. Getting basic unit tests set up with pytest will both help make it easier to start writing more tests and would set the foundations for developing a more diverse testing suite (integration, end-to-end, ..)

Goal

Have a way of running unit tests (that can be run without spinning up the service). The following would meet the requirements of completing this ticket:

• From src/services/ray, have a pytest config, and be able to run unit tests from command line
• For now, just 3 basic unit tests + hello world. Don't bother mocking anything, just find existing classes or functions which have minimal dependencies and create some simple tests.
• Instructions on how to run tests added to README, and informative doc strings.

See the Queue service for an example of what this could look like (ignore the actual tests, they serve more as "proof of concept").