Add tests

[agilgur5]

Noting that I'll likely need to mock out localStorage for tests. Not difficult given that a simple mock can be < 10 LoC (or use some in-memory package).

Unlike Jest, AVA doesn't support mocking out-of-the-box, but we don't really need sinon or too much mocking support for a small mock like this, so should be nbd

[agilgur5]

Make sure to add a localForage integration test to ensure all versions must support it. Any future IStorage interface should also work with localForage's interface and the typed test should guarantee that if it compiles.

[agilgur5]

Still need to add a localForage integration test, but just wanted to note here that per the PR that adds tests, jsdom actually has a localStorage implementation built-in, so there was no need to polyfill/mock that. Though polyfilling it and running entirely in a node environment would likely make the tests run a bit faster as there wouldn't be a need to initialize jsdom anymore then.