Should I User Test a new feature with it's onboarding?

At the moment I do. Reason is, if I never test the onboarding part, how do I know if the execution is correct?
Secondly, if I was to conduct User testing without, then with onboarding, it takes way too long.

But I've been thinking. If I include onboarding on information that’s unnecessary, how will I ever know to strip it?

Does anyone have any good, valid resources that talk about this?

Thanks :)