My recommendation is to set the Fix Version to 1.1, mark it as resolved, and drop a test that mocks twice into the test suite for Grails so that you don't break whatever magically fixes this. I suspect it's some kind of weird consequence of Groovy's updated metaclass implementation.
The Testing plugin then will just have this as a known limitation in 1.0.4. That's reasonable, even if annoying. I'm not sure there's a lot of value in burning time on this with 1.1 being right around the bend.
User list thread is here: http://www.nabble.com/Testing-Plugin-td20590673.html#a20591673