Fixed #23497 -- Made admin system checks run for custom AdminSites.

https://code.djangoproject.com/ticket/23497
I sort of agree that a functional test wouldn't hurt, but the problem I see is that it's not so clear whether or not every test in `admin_checks` needs to be duplicated here (I think the answer is "no", this is just an arbitrary admin test that's used for the functional check).

Maybe we could refactor the test classes so that `AdminScriptTestCase` could be used in `admin_checks` and then there would be a better separate of concerns. If you want to do that, let's submit this without this test now and make it a separate commit.