Improve js unittests

[ollimeier]

          > It would be great to have unittests for such methods, but I don't know how 

I have an idea for starting unittesting FontController and more, and it will be based on the .fontra format: for testing we should implement a class that reads .fontra from disk (Node-only), and is (minimally) compatible with the "remote" font object. We'd need a csv reader for this: we need to check npm. The rest of the data is JSON, and should be trivial to read. (.fontra uses 'unpacked paths' on disk, so we'd need to convert them to PackedPath upon reading, but that's more or less the only diversion from our in-memory object model.)

If you want you can write this up as an issue, so we can turn this into something actionable. Solving this will be very valuable long-term.

Originally posted by @justvanrossum in #1855 (comment)

[justvanrossum]

We already have a test font for this: https://github.com/googlefonts/fontra/tree/main/test-common/fonts/MutatorSans.fontra

[justvanrossum]

We pretty much need to implement a read-only .fontra backend in JS, and then we can start writing unit tests for FontController.