add debug variance link

[lengyijun]

I find src/test/ui/variance/variance-types.rs quite helpful in debugging variance.
I want to put the link into the book.

[ehuss]

Thanks for the PR! I think I would prefer to not include something that points to rustc's testsuite in this way. Also, the reference isn't oriented towards in-depth illustrations into concepts like this. The nomicon is usually the place for things like that, and it has a chapter on variance. If the examples there are not illustrative enough, then I think it would be worthwhile to consider adding more code examples there (inline, not linked).