Add unit tests to solstice-assets

We should look into adding unit tests for solstice-assets. I created this issue to discuss the need for it, and to select a good test runner and assertion library for solstice-assets.

It's become sort of a productivity killer to manually test if a component breaks every time we made a change to its script. This process should definitely be automated going forwards with the help of unit tests.

For test runners and assertion libraries, these seem to be the big three:

• Jest
• Mocha (frequently paired with Chai assertion lib)
• Jasmine

I found this comparison between Jest and Mocha quite useful.