Publish util code tests

Per https://github.com/googlefonts/gftools/pull/191#issuecomment-632239264 there are some unit tests for the util codebase that can be published; however, it may be that since the internal codebase and this one have diverged, we should accept this repo has forked, and the internal project has to be republished - and then this issue becomes a reminder to ensure when that happens the tests are included :)