Add testing

[alwaysblank]

A robust, usable library needs tests to make sure it remains robust and usable.

At the time of release, Jest didn't have good support for MutationObserver, which this library relies on. That may have changed (it was over a year ago, an eternity in webdev time), or we could pivot to another testing library.

Existing work on tests can be found on the tests branch: https://github.com/alwaysblank/drawer/tree/tests