Add Proper Testing Infrastucture to `slicec-cs`

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In slicec we have an entire tests directory with a module of helper functions. These tests are centralized (in the tests directory) and then organized into categories.

Whereas in slicec-cs our tests are more scattered and there's no central helper functions that can be re-used. We should re-structure the tests in this crate to mimic what the slicec crate so writing and maintaining tests is easier.