Replace `mockito` with a mocking library that makes debugging failures easier

[szokeasaurusrex]

It is pretty difficult to deal with debugging failures from the mockito library, as it panics on failure. Also, it replies with a 501 error whenever one tries to hit it with a URL it does not have a response for, and this 501 often calls trycmd to fail.

Let's replace mockito with something which does not panic on failure. This way, we can assert the mocks and print out any errors, so that we can get the errors from both the mocking library and the trycmd/`assert_cmd` libraries. Right now, that is not possible

[linear]

CLI-151 Replace `mockito` with a mocking library that makes debugging failures easier