Add proper tests #325
Description
There are currently a few tests (kinda like unittests), but they are useless and should be removed
Instead, I think there should be functional tests that spawn a child Nvim instance, control it and get its state via message-RPC
What frameworks to choose? I don't know, could be busted, plenary.test, mini.test. Or even pytest with pynvim as I'm experimenting in unnest.nvim