Clean up d.c.sites cache after each test

`django.contrib.sites.models.Site` instances are cached in a global variable `SITE_CACHE`, which can make tests interfere with each other.
For instance, before this commit, tests are failing when run in the following order:
`python tests/runtests.py contenttypes_tests.tests.ContentTypesViewsTests.test_shortcut_with_absolute_url syndication_tests`