Avoid spurious failures in naturaltime tests.

Previous fix didn't cover timesince module, which naturaltime delegates to
under certain conditions. Extended mock of datetime to timesince module, and
switched to a fixed datetime rather than now() for testing, for more
reproducible tests (failures in mock will fail fast, rather than sporadically).

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17038 bcc190cf-cafb-0310-a4f2-bffc1f526a37