Build Unit Tests for Applicable Functions

[Cleptomania]

We already have a number of functions which would be easy and prime candidates for adding Rust unit tests for them. The ones I can think of right now are everything in the math.rs module, as well as everything in the geometry.rs module.

Other things like the HitBoxes and spritelist interaction will be more complicated to test, as they are very dependent on the rest of Arcade and python interop.

[Cleptomania]

I have added a small unit test for the math::clamp function to serve as an example unit test.

[Cleptomania]

We now have complete unit test coverage for the math.rs and geometry.rs modules.

I'm going to leave this issue open to track general test coverage for other modules/into the future.