[DRAFT] Add tests for the voice code

[sapphire-arches]

This is looking like it's going to pull in a whole bunch of nonsense along with the voice code, so that might get submitted as separate PRs first.

[m-m-adams]

Maybe we should try making the core render functions constexpr and then testing at compile time?

[sapphire-arches]

A more gradual approach is likely more suitable here.