Added tests for django.contrib.admindocs -- refs #22015

Added tests which cover `django.contrib.admindocs.views.ModelDetailView` which was seriously lacking in test coverage. This is part of ticket #22015, though the original issue for which the ticket was opened, seems to be already working as expected, with and without this patch. Though this patch does contain a test which covers the issue raised in the ticket, and should catch it in the future should it re-arise.