Add tests

[arcanis]

Because of reentrance & whatnot, the code flow might be a bit hard to follow sometimes, and I would feel more confident if we had tests to validate that the behaviour doesn't break (or at least, that it breaks because bugs are being fixed).

A few things to test:

• A regular action should dispatch a single notification
• Two regular actions should dispatch two different notifications
• An empty batched action should dispatch no notification
• A batched action should dispatch a single notification
• A batched action emitted from within a saga should dispatch a single notification
• A batched action asynchronously emitted from within a saga should dispatch a single notification
• Two actions asynchronously emitted from within a saga should dispatch two different notifications

I still have to think about a good reentrant test.