ex: we shouldn't call Stream._init() in the tests, which is an implementation detail of cuda.core. We should only test public APIs.

It is fine to also have tests for internal modules/functionalities, but they should come after we have coverage for public APIs.

I don't think we have "relational tests" added yet (ex: Device.create_stream() returns a Stream; Stream.record() returns an Event, etc), but let's add them in the next PR.

Originally posted by @leofang in #153 (review)