Hey @Triplis
So you're looking for a test that would result positive if a Sim has a moodlet of a specific mood?
It's not hard to write a test that would do it (https://pastebin.com/QwJQ0jRM), but I haven't found a way to inject that test into the game.
Tests are done via TunableTestVariant class that holds all of the tests inside of it and then that class is initiated by specific tunable types. The only way I would see it happen is if you would runtime edit the TunableGlobalTestSet class (global test of interactions) somehow, but good luck with that. It's probably possible and someone might have done that already, but if you get so deep into scripting, you might as well do the test yourself with a custom class interaction. It's far far easier.
And to your question "Would it be possible to create a test in the future that checks for existing moods of a type", where, I guess, you're asking the developers. Have they ever made anything that is >purely< for modding except the 'Mods' folder that loads the mods? They made great work with allowing for modding, but I don't think they could have planned for such deep modding.