Add unit tests

[whoabuddy]

The framework for testing with Jest is in place on the worker, but nothing has been setup yet.

It'd be nice to start with:

• make sure each endpoint returns the correct values based on:
  • all checks/errors possible depending on data submitted
  • successful response and correct type
• make sure CityConfig options match contract data
  • could be a one-time pass in CI that verifies against read-only functions
  • helps to discover small inconsistencies in hard-coded data without offloading to client
• make sure handler definitions match OpenAPI spec (might be a stretch)

Any other ideas are welcome!