Async and callback tests

[novemberborn]

Given that this package wraps AVA's test & assertion interfaces, I wonder if we need to actually expose all interfaces.

E.g. AVA has callback tests (test.cb(t => { t.pass(); t.end() })). Does that make sense in BuckleScript?

Asynchronous tests need to be marked up as such in the syntax, and assertions over promises require throwsAsync / notThrowsAsync. Does BuckleScript have the concept of promises?

We should do what makes sense for BuckleScript, rather than try and be a perfect wrapper of AVA itself.