test: replace mockery with testthat mocking

[maelle]

I'm suggesting this because I'd like to write an update to my post from 2019, where the real example was this keyring test. 😇 https://blog.r-hub.io/2019/10/29/mocking/#general-mocking

[gaborcsardi]

Thanks! The thing is, I like mockery and the way it works, especially for packages that may be called during a test run. I think switching to testthat is probably fine, but it is just simpler to avoid thinking about this. And possibly keep thinking about it in the future.

[maelle]

Thanks for your response!
For info, the new post draft is at r-hub/blog#171