This PR adds support for concise (init-less) testo.Options calls:

var _ = testo.Options(
    myplugin.WithFoo(),
    myplugin.WithBar(),
)

as opposed to

func init() {
    testo.Options(
        myplugin.WithFoo(),
        myplugin.WithBar(),
    )
}

This is implemented by returning an empty struct{} from Options func, similar to For function.