Add return types to ContentModerationTestTrait

Looking at the traits that implement a createEditorialWorkflow function, they don't seem to be overriding ContentModerationTestTrait. Many are ParagraphsTestBaseTrait.php files that don't have a return value at all. SchedulerUiTrait in lightning_scheduler isn't used in any tests alongside ContentModerationTestTrait. And finally ContentEntitySourceContentModerationTest in tmgmt doesn't interact with the core trait either. I think it'd be safe to add the return type to that function too. But I can also see the argument to not add it so I'll mark this RTBC.

I think it's a good step forwards to do the one method and not the other.

The baseline changes had commit conflicts, but I regenerated the baseline locally.

Committed/pushed to 11.x, thanks!

catch → closed merge request !13030