make tests runnable on Windows

[ceejbot]

The tests right now depend on a lot of not-windows config conditions, driven by two different considerations. The easy consideration to deal with is replacing game engine functions with test stubs so we don't need a running game but can instead feed functions test fixtures. The slightly more annoying consideration is wide strings and OsStrings in general, because make that switch test vs non-test would mess up Rust analyzer on Macintosh. Since I do most of my development on my Mac laptop, that would suck. Maybe introduce a custom config for running tests on Windows? IDK yet.

I note that the tests need to stay runnable on Linux to keep the Github workflows snappy. It takes forever to spin up anything on a Windows runner.