Improve test formatting [draft]

[ty-everett]

While working on Overlay Services, I've found a way to get tests to run without needing to import the cjs files! I've done a lot of the refactors, but there's some edge-cases that still need work. I'll leave a draft pull request, and if it's something people think would be useful, feel free to pick it up and complete the test suite refactors.