Fixed #21774 -- Isolate all test urls from eachother.

This (nearly) completes the work to isolate all the test modules from each other. This is now more important as importing models from another module will case PendingDeprecationWarnings if those modules are not in INSTALLED_APPS. The only remaining obvious dependencies are:

- d.c.auth depends on d.c.admin (because of the is_admin flag to some views), but this is not so important and d.c.admin is in always_installed_apps
- test_client_regress depends on test_client. Eventually these should become a single module, as the split serves no useful purpose.