Maybe I'm interpreting it wrong, but as far as I can tell, mood tests are based on the dominant mood, rather than existing moodlets overall. So, for example, the phenomenon (which I'm now understandin...
Idea: You probably won't be able to tie it directly in with 'statistic_in_category', you might be able to rig it in 'operation_actions' under periodic_stat_change', where you add a buff to the Sim once all the happy moodlets go away. Then you key the interaction to end as soon as said buff is added. Might that work? :)