Update toplevel global actor isolation test
Since we're forcing all of the variables at the top-level to be main
actor isolated, there were some changes to where actor-hops were are to
happen. Additionally, I had to pull of the actor isolation on `a` since
we don't allow explicit global actor isolation. Given this, it doesn't
really make sense to keep `b` around. As a nice change though, we can
call synchronous functions directly in top level code, passing variables
directly.
