Updated tests to stop leaking models in shared AdminSite.

This would break upcoming changes and AdminSite assumptions about
having an app_config for each application that has registered
models.

tests/admin_custom_urls/urls.py

@@ -1,6 +1,7 @@
 from django.conf.urls import include, url
-from django.contrib import admin
+
+from .models import site
 
 urlpatterns = [
-    url(r'^admin/', include(admin.site.urls)),
+    url(r'^admin/', include(site.urls)),
 ]