test effect: remove extra effects from definition time

[timbertson]

Replacement for #53. I think this makes more sense. effectful-test no longer requires the effect at definition time, it just produces a test case that requires the effect. That matches test, which doesn't require io but produces a test<io>.

It was always a little confusing that you could theoretically use one async handler at definition time but a different one at execution time.

And now group doesn't need to assume io, it just adopts whatever test types are used in its body.

I originally thought it might be important to allow additional effects in group bodies, and maybe we'll need to support that later. But I don't know of a use case yet, so let's not guess.