Create unit tests for each public API

[jeremyfowers]

See discussion in #55

Create a test/public_api.py test file that includes 1 or more unit tests per public API. These tests should be as minimal as possible and not rely on any end-to-end actions such as calling the turnkey CLI.

This will help to ensure that we have clear indications whenever the public API is being broken: the public API test will fail, and any changes to the public API test will require a change to the major version number.

cc @danielholanda