Core Data model for test-taking iOS app
Test_Questions and User_Answers are not necessary since they do not hold any attribution and are one-to-one relationships with Test and User, respectively. It is always suspect when you have a plural table name.

However, the model fails to support the ability of a User to take multiple Tests. To fix that, User.testTaken should be replaced by User.testsTaken and made into a one-to-many relationship with Test.

Also, it seems a bit odd that a Test can only be taken by a single User, but if that's your requirement, then so be it.