Implement unit tests for javascript functionality

[ElDeveloper]

Original issue name: plateViewer.js is untested

It would be good to add tests for at least the PlateViewer class, but really anything that may make sense would be good to have properly tested.

[charles-cowart]

Since Javascript implements a lot of use-facing functionality, it would be really good to have at least some basic unit-testing for plate mapping at the least. We may find the source of some of our intermittent errors here.

[wasade]

This is an ongoing issue partially addressed with the "Establish a Javascript unit testing framework" milestone. Removing from that milestone as fully resolving this specific issue was not a requirement for completion of the milestone.