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.

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The eventual goal is to port slicec-cs to C# (See #3978), so adding tests to the Rust implementation is pointless now.