Decide a test structure

[ggonzalez94]

So far we've been fairly loose in how we organize tests, mostly putting them under a single file per contract. We should make a conscious decision that improves organization, but is not overkill(if in doubt we should always defer to the simplest structure).

Test organization

@LeoPatOZ suggested using this state driven approach. Foundry has also some guidance on how to best organize your tests.