tracking: improve test coverage

Achieving 100% test coverage in this repo isn't critical but would be a nice thing to have. It's also an indicator, of some degree, of the quality of the repo. For anyone wanting to help on this front, you can go to [this repo on Codecov](https://codecov.io/gh/denoland/std) and choose an area of the codebase that you can add tests for until said area reaches 100% test coverage.