It's his Mentor's responsibility to Teach him/her. How well are you teaching him/her HOW to test. Are you pair programming with him? The Junior more than likely doesn't know HOW to set up a good test for xyz.
As a Junior freshout of school he knows many Concepts. Some technique. Some experience. But in the end, all a Junior is POTENTIAL. Almost every feature they work on, there will be something new that they have never done before. Sure the Junior may have done a simple State pattern for a project in class, opening and shutting "doors", but never a real world application of the patterns.
He/she will only be as good as how well you teach. If they were able to "Just get it" do you think they would have taken a Junior position in the first place?
In my experience Juniors are hired and given almost same responsibility as Seniors, but are just paid less and then ignored when they start to falter. Forgive me if i seem bitter, it's 'cause i am.
