Sys.process exit test shouldn't actually exit

[phated]

We should use our "wasi polyfill" to test sys.process exit, which would allow this test to work on Windows (or a different wasm runtime).

[ospencer]

Just to clarify, switching to, say, WAMR, would cause the test to fail because we wouldn't be shelling out anymore (so we'd exit the current process, that being the tests themselves).

But of course we're going to avoid this entirely 😄

[phated]

This hasn't been solved for the wasm runtime case, but it is temporarily worked-around by #1223 (for our Windows testing needs)