add unit tests

[thescientist13]

based off the work done in [component-simple-slider]
(https://github.com/ProjectEvergreen/component-simple-slider) to add unit tests, unit tests should be added to this project now.

✨ Bonus points: Submit back to the wiki docs if any interesting testing patterns / techniques are introduced.

related: ProjectEvergreen/project-evergreen#1

[thescientist13]

Karma has been added to CEA, so it should be used here