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

https://code.djangoproject.com/ticket/23497
Original tests were in `tests/check_framework/tests.py`: https://github.com/django/django/pull/3227/files#diff-dc8c55ebf8072120b505dd8dade78efcR341

I've simplified the test case and moved it to here. I agree that the place is not ideal, but I think it would be good to have a more functional test in addition to a unit test in `tests/admin_checks/tests.py`. However, since it's not a critical feature or bug, the `test_custom_adminsite` test in  `tests/admin_checks/tests.py` would be enough. So, I can remove this test if you want.