Add test link to subtype

[lengyijun]

I find src/test/ui/variance/variance-types.rs quite helpful in debugging variance.

[ehuss]

As mentioned over at rust-lang/reference#1228 (comment), I don't think just adding a filename here is a good idea. There's no context as to what that means and doesn't flow with the existing text. If there are specific points missing, then they should be added. Also, this chapter is being rewritten over at #340, so I think it would be good to hold off on any changes until that lands.