Fixed #10414 -- Make select_related fail on invalid field names.

https://code.djangoproject.com/ticket/10414

This is basically my first non-trivial contribution to Django, so it might not be perfect. I'll be happy to update the PR as required. :)
Good point, but I didn't want to pollute the class with a random attribute since most other test in the class does not need it. How about making a `SelectRelatedValidationTests` class for these new tests (and then using an attribute as you suggest)?