Use mockr to write unit tests

[isteves]

From #13:

For your issue of wanting to test your processing code with a formal unit test, your intuition is right on. The way people like to do this is to make what's called a mock. It's basically what you said: save a copy of some response to some function call that usually is slow/can fail/etc and test that. This makes unit test unit tests and not integration tests!

I haven't done this directly with testthat, but there appears to be support: https://github.com/r-lib/testthat/blob/master/R/mock.R. Also see https://www.mango-solutions.com/blog/testing-without-the-internet-using-mock-functions.

[isteves]

Possibly helpful resource: https://enpiar.com/talks/api-in-r/

[amoeba]

https://github.com/ropensci/vcr just got httr support so this could be away to go too. Not sure how to choose between available options yet. Yet!

[amoeba]

I took a look at this during the pre-1.0 refactor and tried out vcr. I ran into an issue I couldn't work around that may have something to do with RT's use of session cookies and our use of httr's built-in cookie caching functionality. I've ping Scott Chamberlain (creator of the vcr R package).

At this point, the API wrappers with more complicated logic make use of helper functions, some of which have been factored out and are unit tested separately. Otherwise, we have integration / end-to-end tests set up to run via GitHub Actions and they can also be run locally with Docker.

Doing more refactoring and/or integrating mocks or vcr may be of benefit but I think, at this point, the package is pretty well tested so I'm going to move this to the next milestone.