tests do not cover fonts

### Expected Behavior

Automated tests should cover all major aspects of the code in this module

### Actual Behavior

The code in `font-inliner.js` is not covered in any test. This means if we change the API for `scratch-render-fonts` (as we did recently) in such a way that `font-inliner.js` no longer works, the tests will still pass and not call attention to the problem.
