### What does this PR do?
Lets us write and run unit tests directly in Zig.
Running Zig unit tests in CI is blocked by https://github.com/ziglang/zig/issues/23281. We can un-comment relevant code once this is fixed.
#### Workflow
> I'll finish writing this up later, but some initial points are below.
> Tl;Dr: `bun build:test`
Test binaries can be made for any kind of build. They are called `<bun>-test` and live next to their corresponding `bun` bin. For example, debug tests compile to `build/debug/bun-debug-test`.
Test binaries re-use most cmake/zig build steps from normal bun binaries, so building one after a normal bun build is pretty fast.
### How did you verify your code works?
I tested that my tests run tests.
