I've just gotten started with JUCE and was wondering the same. I think it should be pretty simple to add a build target that just compiles a command-line binary with a main that runs your unit tests - I figure this is a lot easier than actually having to build test functionality into a VST target.
The problem is, I tried adding such a target in XCode, but then as soon as I re-ran the Introjucer it deleted my new unit test target. So I guess that's not much of an option :-/