Add JS tests

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Not adding this to the 2.0 milestone because the package is simple enough that I feel okay about no tests (and I have no experience writing JS testing, so it may be a long road), but tests should definitely be implemented to make future development/enhancement quicker and safer.

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I'm putting this on hold because I'm not quite sure how to proceed. The module relies very heavily on MutationObserver, which doesn't appear to be accurately replicated in Jest. I think the way forward is probably to switch over to Mocha, or something that will be running this stuff in an actual headless browser, but I'm not relishing the idea of re-writing the tests I've already written. It is a goal, and only I would ideally like to make at least some progress on before 2.0 release.

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FYI existing progress can be found on the tests, branch: https://github.com/murmurcreative/module-drawer/tree/tests