Audit and convert tests to test/unit (minitest)

:construction: WIP :construction: 

This PR converts the RSpec tests to minitest. Most of the tests were translated over, but I'll make inline notes where I modified or removed tests for clarity.
I'd like to remove flexmock and only use minitest/mock here, but I had some difficulty converting this test. It's not high priority, but if someone is interested in tackling it, feel free to talk to me.