Refactor tests to look more like other libs

[tstirrat15]

Probably use authzed-dotnet as a template. We'll still want to test both the promise and non-promise implementations, similar to how the python lib does both async and non-async, but it'd be nice to standardize the tests a little bit.

[radik]

Hi @tstirrat15
Here is a draft PR with re-written tests - #237
Let me know if I'm going to a wrong direction. Otherwise - feel free to assign the issue to me, and I'll finish PR.