[Checkers] Interned test cases

[Mahmood Ali]

> I took a quick look at the failing test, and it seems to be because  
> SimpleSubtypeRelation.isSubtype doesn't call asSuper(). Mahmood and  
> I were talking recently about whether isSubtype() or its caller  
> should use asSuper() (i.e., whether "uses asSuper()" should be part  
> of the spec of isSubtype()) and I don't remember if we came up with  
> a good answer. However, I'm going to have SimpleSubtypeRelation use  
> asSuper() (like IGJ does) for now, and we can change it later if  
> necessary.

The bug actually was due to us specifying that the raw type was a  
supertype of the generic type.  I just submitted a check in to remove  
that relationship from the return value of factory.directSupertype().   
Checker.isSubtype(), however, need to take that into consideration.   
All tests passed in my local repo.

- Mahmood