I'm kinda growing to like my idea the more I think about it. You're right that it adds 3 characters to the common case, but lisp has a long tradition of empty parens in various places. Saving characters shouldn't be a high priority, IMO. Paul Graham's notion of conciseness counts tokens, not characters.
But yeah, happy to see what other ideas we can come up with. I think the setup keyword above is worse; lisps don't tend to have keywords that aren't functions or macros. Then again, test is already a keyword that's not a function.. Hmm, I like it better if you indent it like this:
(setup a 1
(assert-same b (+ a a))))
The benefit of this approach is that it makes the syntax seem extensible. It's obvious how new features should be added.