Test for manual_evaluation exercise messages behavior

[luchotc]

Creating a draft PR for current issue with messages.

This is due to exercises default_scope 😕.

[luchotc]

@flbulgarelli I'm not sure which is the best way to tackle this. We could limit messages like .where(..exercises: organization.exercises) and that would do the trick, but it feels like a nasty patch 😕

[flbulgarelli]

Hi! I think I need a little more context here 😅